One sided love/arranged love/triangular love

I am wondering why many a times love is one-sided? One person feels very deeply, however the other claims to feel nothing and sometimes ends up causing a lot of hurt to the other person? Sometimes the other person loves someone else?  Why does these situations happen?  What lessons does a one-sided love teach? A lot of Bollywood movies depict triangular love stories e.g. Kuch Kuch hota hai, Dil to pagal hai etc, Ofcourse these movies are exaggerated, but do they say something deeper about life?

Secondly, does everyone expereince true love - does everyone qualify for true love? I see a lot of people
getting married, especially arranged marriages, where people are matched on education, family's social standing, income (or the potential to provide a good life), looks, age etc ?  Are these marriages based on convenience or is there some love beneath the obviously love-less surface.