To understand why we need to look at Islamic based on history and the Quran:

So Mohammed was born in the town of Mecca. At the time most of the people living in the east were Pagans. Mohammed made trips to a cave where he prayed and one day got a vision from the angel Gabriel. Telling him about Islam. He then went back to his tribe and told them about what had happened. His tribe and his uncle was a big supporter of what Mohammed had to say. It was said that Mohammed went into a trance and started to recite the Quran. This made most of the tribes people mad as they considered the belief of one god as blasphemous. The meccan Tribes had enough of these teachings so they tortured, burnt and left Muslims to starve in retaliation. Later his Uncle and wife died. Later a town which was undergoing major rivalries and fighting invited Muhammad to settle the dispute and in return they would provide refugee. For Mohammed and his followers leaving Mecca, their home town was a big thing and this voyage is said to be the first day in the Islamic Calendar. As his mediating started to work this town would later become the city of the prophet, Medina. Another quick reason why Islam was hard for people to take in was because it preached that God had revealed his word to mankind many times, through Jesus, Moses, Abraham (since the Quran considers Christians and Jews to be people of the book) but mankind went astray and the Quran is Gods attempt to set things right again. Later Medina started to become a Islamic city and Mohammed's friend, a freed Abashian slave named Bilal gave the first call to prayer in Medina. After hearing about what was going on in Media all out tribal warfare ensued. The people of Mecca declared war on Islam and attempted to attack Media. After Mohammed tried to talk it out and failed he was forced to fight. The Muslims were able to muster a force of 313 against the 1000 strong tribe members. Mohammed's forces were mostly boys and old men. Somehow Mohammed one battles and the different Meccan tribes saw gods hand in Mohammed's victories and started to convert to Islam and join the fight. Eventually the tide had turned and Mohammed then attacked the city of Mecca. After fighting for a month, Mohammed and his people were able to return home. The people of Mecca feared Mohammed as they thought that they would be massacred (since at the time there was little courtesy given to the losers of tribal fights) but he let the very people who tortured his followers to live. But some of the Meccans fled to the Kaaba in Mecca where they had all their idols set up (since all tribes believed in a different idol) Muhammad's men circled the Kaaba seven times and then smashed all the idols in front of the Meccan tribes. This action was seen as blaspheme as Mohammed was telling them that what they believed for so many years was wrong. This action has the same meaning as for example Moses smashing of the tablets of the law. After this Muhammad had died and was carried to Jerusalem where he rode a magical steed to heaven. After this Islam had boomed under new Caliph (successor to Mohammed) Islam had to do very little conquering as tribes people all over the middle east converted to islam, the Byzantine Empire and their territories were being mismanaged so they accepted Islam as a Liberator to Byzantine rule. The expansion was stopped during the battle of tours when Islam attempted to convert France. In Islam Muslims do not believe Muhammad to be the son of god he was a mortal and lived a full life that's why Muslims believe that on the day of judgement Jesus Christ will be brought back just like the Christian Belief. The ironic thing is that Islam and Christianity are the most similar religions yet we have fought the most with each other. The Mosque in Damascus was a Mosque and one day a church for the Christians. The dome of the rock built in Jerusalem is amazing its hard to imagine 6th century people building such a thing.

1. Terrorism is above all murder. Murder is strictly forbidden in the Qur’an. Qur'an 6:151 says, “and do not kill a soul that God has made sacrosanct, save lawfully.” (i.e. murder is forbidden but the death penalty imposed by the state for a crime is permitted). 5:53 says, “… whoso kills a soul, unless it be for murder or for wreaking corruption in the land, it shall be as if he had killed all mankind; and he who saves a life, it shall be as if he had given life to all mankind.”
2. If the motive for terrorism is religious, it is impermissible in Islamic law. It is forbidden to attempt to impose Islam on other people. The Qur’an says, “There is no compulsion in religion. The right way has become distinct from error.” (-The Cow, 2:256). Note that this verse was revealed in Medina in 622 AD or after and was never abrogated by any other verse of the Quran. Islam’s holy book forbids coercing people into adopting any religion. They have to willingly choose it.
3. Islamic law forbids aggressive warfare. The Quran says, “But if the enemies incline towards peace, do you also incline towards peace. And trust in God! For He is the one who hears and knows all things.” (8:61) The Quran chapter “The Cow,” 2:190, says, “Fight in the way of God against those who fight against you, but begin not hostilities. Lo! God loveth not aggressors.”
4. In the Islamic law of war, not just any civil engineer can declare or launch a war. It is the prerogative of the duly constituted leader of the Muslim community that engages in the war. Nowadays that would be the president or prime minister of the state, as advised by the mufti or national jurisconsult.
5. The killing of innocent non-combatants is forbidden. According to Sunni tradition, ‘Abu Bakr al-Siddiq, the first Caliph, gave these instructions to his armies: “I instruct you in ten matters: Do not kill women, children, the old, or the infirm; do not cut down fruit-bearing trees; do not destroy any town . . . ” (Malik’s Muwatta’, “Kitab al-Jihad.”)
6. Terrorism or hirabah is forbidden in Islamic law, which groups it with brigandage, highway robbery and extortion rackets– any illicit use of fear and coercion in public spaces for money or power. The principle of forbidding the spreading of terror in the land is based on the Qur’an (Surah al-Ma’ida 5:33–34). Prominent [pdf] Muslim legal scholar Sherman Jackson writes, “The Spanish Maliki jurist Ibn `Abd al-Barr (d. 464/ 1070)) defines the agent of hiraba as ‘Anyone who disturbs free passage in the streets and renders them unsafe to travel, striving to spread corruption in the land by taking money, killing people or violating what God has made it unlawful to violate is guilty of hirabah . . .”
7. Sneak attacks are forbidden. Muslim commanders must give the enemy fair warning that war is imminent. The Prophet Muhammad at one point gave 4 months notice.
8. The Prophet Muhammad counseled doing good to those who harm you and is said to have commanded, “Do not be people without minds of your own, saying that if others treat you well you will treat them well, and that if they do wrong you will do wrong to them. Instead, accustom yourselves to do good if people do good and not to do wrong (even) if they do evil.” (Al-Tirmidhi)
9. The Qur’an demands of believers that they exercise justice toward people even where they have reason to be angry with them: “And do not let the hatred of a people prevent you from being just. Be just; that is nearer to righteousness.”[5:8]
10. The Qur’an assures Christians and Jews of paradise if they believe and do good works, and commends Christians as the best friends of Muslims. I wrote elsewhere, “Dangerous falsehoods are being promulgated to the American public. The Quran does not preach violence against Christians. Quran 5:69 says (Arberry): “Surely they that believe, and those of Jewry, and the Christians, and those Sabeaans, whoso believes in God and the Last Day, and works righteousness–their wage waits them with their Lord, and no fear shall be on them, neither shall they sorrow.” In other words, the Quran promises Christians and Jews along with Muslims that if they have faith and works, they need have no fear in the afterlife. It is not saying that non-Muslims go to hell– quite the opposite. When speaking of the 7th-century situation in the Muslim city-state of Medina, which was at war with pagan Mecca, the Quran notes that the polytheists and some Arabian Jewish tribes were opposed to Islam, but then goes on to say: 5:82. ” . . . and you will find the nearest in love to the believers [Muslims] those who say: ‘We are Christians.’ That is because amongst them are priests and monks, and they are not proud.” So the Quran not only does not urge Muslims to commit violence against Christians, it calls them “nearest in love” to the Muslims! The reason given is their piety, their ability to produce holy persons dedicated to God, and their lack of overweening pride.