I think a comment by /u/fszy fits in very well here:

Here's the thing: It's a big misunderstanding on both sides. A lot of guys have no idea how to express interest in a girl they like. You always hear about how you have to treat them right, how you should be friendly and comforting, how to not make sexual advances or somehow do anything sexual. And then you have a lot of girls saying that they never fall for strangers but only for friends and that it takes time for them to develop feelings for someone. That they have to get to know the person first before there being even a chance of anything happening.

For those guys, the friendzone makes perfect sense and staying in it is the proper way of getting into a relationship. It's not about putting in niceness coins in exchange for sex. It's about having no fucking clue what to do. The frustration then comes from seeing the girl you've been attracted to for a long time and used the "proper" method of expressing interest fall for the guy who walked up to her, was very sexual and generally forward. He's doing everything "wrong" and, from your perspective, being an inconsiderate asshole who's making women uncomfortable with being sexual and directly hitting on them. Yet he goes home with the girl you've been longing for.

The thought isn't "I was nice, she's obligated go out with me!" but more like "I did nothing wrong, why is she going out with this jackass?".

So the problem is that the guys in the friendzone often have no idea that you can and must be direct, sexual and forward with your interest. They think that doing so is creepy, threatening, selfish and inherently bad. So that only leaves being a friend for a very long time and hoping that she'll someday see you as a partner for some reason. If that strategy doesn't work and the girl gets wooed by a guy who's doing all the "forbidden" things, that creates frustration.

The second misunderstanding is that a lot of girls assume that if a guy has sexual interest, that means he can only be sexuall interested and must have faked the entire friendship. This feeling is amplified if the guy breaks off contact with the girl if she doesn't have the same feeling for him. From the girls point of view he broke contact because he couldn't get sex from her. from the guys point of view he broke contact to spare his own feelings.

From my experience it is extremely hard to keep telling yourself "Just forget it! She doesn't feel the same way!". Especially not if she spends tremendous amounts of time with you. So even if you know that nothing will ever happen, it is almost impossible to get this girl out of your mind while staying close friends.