Just liking someone is not enough to become friends, one needs to establish some kind of ritual where you can hang out together. Over the years I have seen and been part of many rituals. For example when in school, I wanted to be cool like the repeaters of my class, though I never failed a year, I use to always sit on the last bench and hang out with the repeaters. In college, I would like to hang out with guys who started smoking and drinking early.
Then after I got in to Salsa, I had my Salsa friends who I could hang out and dance. Then there are friends who you would like to watch a movie or music concert. So as you grow up, the ritual change and so do the friends. Friends, who were active part of your life, can take a back seat, just because you no longer are in to the common ritual.
Some interest remains constant like music, art, reading, so any friendship which evolves around these hobbies, will remain forever.
It makes one sad, about so many people you really liked but you could never become good friends, because of a lack of a common ritual.