First, here's an article based on statistics from a database compiled by the Guardian. I believe there are multiple factors involved, but three primary ones:

(1) the availability of weapons;

(2) the militarization of police departments; and

(3) the Supreme Court jurisprudence granting significant immunities and recognizing broad discretion afforded to officers for acts in the course of duty.

With respect to the availability of weapons, there are many studies that show lower rates of gun-related violence and deaths in countries with strict gun laws. Opponents of strict gun laws cite the Second Amendment of the US Constitution as providing a personal right to own weapons and often dispute the applicability of studies of other countries to the US. I believe, though, that the studies are applicable and that Americans of all stripes agree that increased availability increases consequences when it comes to everything but weapons. For instance, Americans have little trouble supporting strict drug laws and laws against pornography and gambling, and they cite the increase in negative consequences that follow from greater availability of these things. Weapons are no different, but there is a very powerful gun lobby in the US, which, combined with the Second Amendment argument and the culture of objectifying and almost worshiping guns, leads many Americans to refuse to accept the obvious correlation between more availability and higher rates of consequences.

The militarization of police departments has happened in part as a response to the violence associated with more weapons in the hands of criminals, and in part because the US Defense Dept started giving surplus military equipment to police officers a few years into the wars in Iraq and Afghanistan. Police develop a bunker mentality when criminals are so heavily armed that officers have to worry about being shot during even routine traffic stops. This bunker mentality separates the officers from the community with further perpetuates the ‘us’ versus ‘them’ mentality and without trust between the two, officers are far more likely to view every interaction with civilians as a potential deadly situation. Add to that the battle dress uniforms, the automatic rifles and even tanks that police departments receive from the DOD and ultimately the relationship that began as police being civic protectors changes to police acting as embattled soldiers amid a hostile populace.

Finally, the conservatives on the US Supreme Court have repeatedly narrowed suspects’ rights and deferred to officers’ judgments in the field over the past 25 years. For instance, there have been cases where the majority held that officers’ testimony is entitled to added weight as evidence because of their ‘trusted’ position as keepers of the peace, a string of cases granting officers more and more leeway in searches and seizures based on concerns for safety even though the same searches would have been held unconstitutional only a few decades prior, cases where officers have used what many would consider excessive force and have been exonerated because of perceived threats, no matter how tenuous.

There are other factors, but the confluence of these have led to a police culture that sees itself in a more militarized role, distrustful of the civilians they are supposed to protect and more quick to use and justify violence against those civilians. In response, many in the civilian population distrust police and treat them with fear and disdain, which perpetuates the negative relationship.

There are many programs based on the theory of community policing, where officers are taught to interact with civilians and to view themselves as part of the community they serve. These programs emphasize assisting community members and de-escalation of confrontations. They appear to work well to rebuild trust and reduce violence towards and from police, but they are expensive and require a commitment from police leaders to institute properly.