There are several reasons why people end up in the friend zone, but the one I hear about most often is this:
you've tipped the scales of attraction vs comfort to the comfort side. They have become too comfortable with you, and as such, you are no longer deemed "relationship material." You become like a brother. Or a lamp.
Let's just assume for the sake of this post that the concept of
friends with benefits does not exist.
Take a look at what this man is doing: he is balancing. It's exactly like that. You meet a girl, she thinks you're cute, and the countdown timer begins. Wait too long and the scale shifts: she loses interest and you lose your chance for a while. So, you have to keep inching it in the opposite direction by showing her that you're interested in being more than just friends. Strike while the iron is hot, because that window is closing, and it won't wait around. Be adventurous and flirtatious. Take them out, but
don't be there at their beck and call. You're a man, not a dog. You have your own life to live, you have your own problems, and you can't be dealing with somebody else's emotional instability. I'm not saying to treat women like shit, but don't be there to pamper their asses. Think of it like a mystery: the best ones keep you in suspense, they keep you guessing. They never reveal themselves completely. Be like that: intrigue them, make them wanting more.
The friend zone sucks, there's no doubt about that, and while you can look up all of the advice in the world on how to avoid it, it pales in comparison to the actual experience you get from finding yourself in there.
Now for my female readers out there, let me offer you some advice: a lot of guys
SUCK at getting the message. Women are trained to let men down gently and vaguely, "I'm not ready for a relationship right now," "I think it's too soon for me," repeat ad nauseam. This gives them the idea that, if they wait around long enough, you'll finally be "ready." Don't do this. Let them know "Hey, I don't like you like that." Is it blunt? Yes. Will it hurt? Most likely, provided they don't have skin made of titanium, but
it is far, far better to tell them this way, than to imbue them with the sense of false hope that you will someday be available to them.