Well, theoretically the reasons for a person to fall in love with another person changes with age. I read in a book a while ago that the only pure love is the one that happens when we are very young, prolly teenagers, bcz our interests and expectations are not developed yet, therefore "love" is genuine .When we grow, we become more self aware of what we want in a SO ,so we tend to fall in love with the ppl that most fit the profile we have in our mind for that person. On the other hand there are ppl who believe that the only true love is the selfless one. The one where u love someone unconditionally . I personaly believe those ppl are self destructive. ( unless we talk abt the love of a mother for her child)
I gotta say, i'm not a teenager therefore I pretty much follow in the category mentioned above.