I feel like this is going over a lot of what has already been said, but to simplify everything...
"Being friendzoned" is a term used to conveniently describe the situation wherein one person has a romantic attraction to another, but the second person has no romantic feeling for them.
What the term is often used for however is a strange view that being friends with someone entitles you to start a relationship with them. This is untrue, and regardless of the strength of your feelings for them you cannot force them to feel the same way.
There are many reasons for this, not least among them physical attraction - perhaps the other person simply doesn't find you aesthetically pleasing... or maybe isn't even attracted to your gender. You can't change that. Furthermore, they may seek personality qualities which you do not possess - for instance someone might want someone with an innocent, idealistic sense of humour and you possess a cynical, sarcastic one.
In the end, it comes to the point where you must either accept that they do not find you attractive, or cease contact with them. If the latter is your preferable option, you probably weren't that good friends with them to begin with.
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Also, as a side note - I didn't use any gender-specific terms there. Why? It is not an exclusively guy-likes-girl situation, and the same thing has happened to many women over the years as well.
...We just used to call it 'having a crush on someone', rather than attaching an odd name to it.
