Given my past experience and recent encounter, I conclude that there is such a phenomena named "Friendship-Love-Friendship Death-trap" a.k.a FLF Death. Here is how it goes:
Firstly, a boy meets a girl, and they become friend. Then the boy kinda like the girl, thus he decides to make his move and tackle the girl. But at this point of time, only the boy has feelings towards the girl, but the girl might not have the same feelings. The girl might be kinda like him as well and feel comfortable with his company, thus did not reject him, neither did she accepted him. The boy decides to play cool and take it easy, thus he didn't rush things (he couldn't either, even he wants to). As times passed by, the boy find it difficult to deal with all the emotions and feelings, as he likes her but there is no venue to unleash his feelings. To him it is time to move on to the next stage, to be in a relationship and be lovy dovy. Since he can't move forward with the girl he likes (as she had not like him yet), he have no choice but to suppress his feelings. The best way is to kill all hope, thus treating her only as friend, with no hope or expectation for anything more intimate. Then he starts to feel more comfortable being with the girl, putting all intimate feelings aside. After sometime, the girl started to like the guy, as she understand him better and did spent quite sometime together. She enjoys his company, his openness and his sharing. At the time when the girl opens her heart, the boy had already closed his heart. He had stop the flow of feelings as it hard been difficult for him to handle in the past, liking someone without expectation. He stop being sensitive to the girl's feelings, and treat her as a close friend (though close, but just a friend). Even though someday he realise the girl did have feelings for him, but he found that it is kinda difficult to light up the old flame and bring up the feelings again. The feelings had changed form, loosing its hope and expectation. Thus, they will always end up as just friend.
Perhaps a guy can easily fall for a girl, thus liking her and show his affection. For a girl, maybe she requires more understanding for built-up of feelings, thus only like the guy at a later stage. If the gap between the feelings built-up for a guy and a girl is too large, then they would easily fall into FLF Death. You start with a friendship, then the feelings developed (at different time) and it end up as friend again, with no returning back or repeating cycle. Guy and Girl are more easily in sync when they are young, as both party are eager for love and can start a relationship easily. As we mature, we start to think a lot, letting our logical mind work harder than the emotional side (we are no longer that naive, and we learn to protect ourselves as this is a dangerous world after all). The gap started to grow and it started to favour FLF Death.
I still remember the 1st occurrence of FLF Death for me as during secondary school with the first girl I like, Fun. I courageous admitted I like her, but the relationship did not start off instantly. We start by some small dates, many phone calls and I even manage to befriend her mom (their relationship is close, and her mom is an open-minded person). Somehow, we just didn't step into a serious relationship, though we are very close. Though I felt that she is ready for me at a later stage, but I am just not motivated to act on it. Perhaps the feelings had changed, from intimacy to friendship.
Beware of FLF Death, as feelings developed which are not in sync with each other shall not bear any fruits :)