"Nice guys always finish last."
This phrase has been uttered under the breath of countless men with "Nice Guy Syndrome." Men with this syndrome often make the mistake of thinking that being a "nice guy" is enough of a quality to get them laid. This common misconception is flawed for so many reasons. Being a "nice" person is literally the easiest human trait you could pick up. It takes almost zero effort to be nice. Also, are you really a nice person if you're only kind to people to obtain love or sex?
Generally, when men claim they are "friend-zoned," they're literally just complaining that women can say no. It's implying that there is something owed to the man, simply because they're a decent human being. This is not to say you shouldn't be a decent person; you should be a kind person simply because you should. However, if the best way to describe someone is "nice," that probably means that person is boring. Maybe the reason why girls are dating other men instead of nice guys is because the other men have more to offer. Instead of their personality being based on basic human decency, men who succeed in relationships tend to be funny, interesting, smart, etc. Maybe try developing a real connection with people and becoming an interesting person instead of being mad that people aren't attractive to your bland personality.
Now, there's a difference between being "friend-zoned" and falling for someone and that person not reciprocating. The difference is whether you move on afterward, or shame that person for not feeling the same way as you. For some reason, many people cannot understand this. When men hear the phrase "No thanks, I'm not interested," they don't take it to heart. They think it means "convince me," when really, it means "No thanks, I'm not interested." When they finally get it through their thick skulls that maybe they don't have a chance with a woman, they immediately prove they aren't, in fact, nice at all, and call that woman a bitch, simply for saying no.
If you don't classify yourself as a "nice guy," congrats. Now, understand it's your job to call guys out when they claim they're in the friend-zone. Let them know that no one owes them anything for being a good person, and being a good person should just be expected. If you're only in a friendship hoping for the implication of sex, then maybe you shouldn't be interacting with that person. Learn to take "no" for an answer and grow up. It all comes down to maturity and learning how to handle rejection like an adult.