Nice Guys have this "Center Of The Universe" mentality, in order to hide the fact that they still have insecurities and issues to overcome as a person. Basically, I like to compare this mindset to badly written Mary Sue characters;

• Anything they do is the right thing to do, even when you really think about it, is a terrible thing to do.

• Anything that gets in the way of his desires is immediately evil, and can only have the most terrible of intentions, regardless of what they actually do. Also, because they're so evil, the Nice Guy is justified to do things that, again would be considered evil anywhere else.

• The Girl female must always be interested in the Nice Guy, and, if not, it's considered getting in the way of his desires, which lead to rule #2

So, for instance, if he spends all his time doing actions that are commonly considered very stalkerish, that's fine, because he's just being shy, and you should have sympathy for his poor plight. However, if someone else goes up to the girl he was creepily shyly watching intently and asks her out, he's immediately evil and wants to rape and murder her, possibly at the same time because he's JUST THAT EVIL

There's also the unfortunate idea that tends to spawn during the teenage years that society has a predetermined set of things that every man/woman must enjoy intensely, and that stepping outside of those boundaries by enjoying something out of the norm immediately believes that Society as a whole will cast you out, and will use every opportunity they can to tell you how "Autistic" and "retarded" you are (which, as a person that enjoys a lot of neckbeardy things, I can say that that is simply not true, at least not entirely. People are a lot more tolerant than given credit for, as long as moderation is used in regards to it as a topic.), so they view someone who might just be trying to give them advice without a drop of hate in the world as someone who's confirmed to society trying to pull them in as well.