Often in the romantic genre there is this idea of the best 'friend'. Usually the person is of the opposite gender of the main protagonist and will fill the role of 'perfect love' at some point. Now you can usually tell pretty quickly if that's the type of romance something is going to be because there's usually a best friend of the same gender when the love interest is going to be someone else and this is not what is used.
I find the 'friend' line an interesting one. There are times where you wish your best friend was interested in you, at least I know there was for me. Then there were times where my best friend was interested in me except one kiss convinced me I did not see him that way. We have this tendency to want to be with someone who is also our best friend. I think it's why so often we start out as wanting to be 'friends' first. (at least from the woman's perspective, don't know about you guys.) We want someone who understands us the way a best friend does and loves us both in spite of and because of our faults. It's a good thing. But why then do we tend to overlook people in that 'friend' zone?
Honestly? It seems to usually be because there was no chemistry to begin with, especially when the friendship was formed far before 'chemistry' existed within such young children. Now this can be because we haven't thought of a person that way for whatever reason, usually because they've just always been there and it never occurred to use to change our views from that really young age. Sometimes it's because one or the other just isn't attracted to the person (and yes it happens, it's not supposed to be an insult even though it's hard. Not everyone clicks that way, even if they want to.) There's nothing wrong with either person at that point, but it just doesn't work.
An interesting example of the 'friend' zone with one wanting more and the other not seeing it right away is P.S. I Love You. Very sad movie, one of my favourites, despite a certain actor I'm not very fond of, but very well done movie. Watch it and watch the development of the 'friend' relationship. *spoilers to follow* It clearly demonstrates the differences between the female protagonist and the initial male who comforts her after she *spoiler* loses her husband. Eventually the 'friend' demands attention from her and they kiss, but they are both relieved in that when they do kiss it does nothing for either of them. That's a happy way to do it, as is the kiss where it means something for both of them. The sad ones happen when something happens for one, but not the other, which isn't as common but does occasionally come up in books and film to create heartache to fill for one or the other. It can be useful.
The 'friend' zone isn't all that scary a zone to be in if you can handle the uncertainty it can cause. It can also be a fascinating relationship to tackle when you're writing something. Just keep in mind that the 'friend' zone for the person who continues to like the other when it's uneven makes both uncomfortable more than likely. At least it does if the main protagonist is aware of the other person's feelings, and especially if they were genuinely friends. Friendship isn't something we like to give up easily when there's no really good reason. Seeing someone else in pain doesn't make it easy on a person, that's for sure. And there's so many fun ways to throw things off and up and it's always fun to not have them kiss, just to have someone reject the friend only to later realize that they really were attracted to them after all. It leads to some interesting and fun dynamics.