If I'm mildly attracted to someone, I find it alright if that person is attracted to me too in some respect (doesn't need to be sexual), but doesn't actually want sex or to be in a relationship with me. The reverse scenario I find quite comfortable as well so long as it doesn't result in hurt feelings. I think maybe part of that is that there's still a mutual emotional validation and a wanted nourishing exchange of emotions of sorts, which is adequate for me. It gives something beyond the kind of friendship I could have with someone who isn't attracted to me in any way. I actually tend to prefer being mildly attracted to male or lesbian friends as well, but feel less of a need for some degree of symmetry in those feelings from them and can enjoy the rather one-sided adoration from my direction. This also applies to some homosexual/bi men or heterosexual/bi women where I sort of know I'm not their type or its just not in their nature to have low intensity attractions.
I suspect around two thirds of the friendships I had with female individuals in the past included them being sexually or romantically attracted to me during some period, a thin majority of those were partially reciprocated on my part at some point, but not enough that I actually wanted to be deeply involved with them in such ways. I have the casual warmth / friendliness often exuded by INFJs. When my attraction levels for someone is configured, their attraction for me is somehow mixed into the formula, which leads to me pretty much never having had a strong unreciprocated attraction to someone (unrequited love).
TL: DR I like mingling some emotional aspects commonly found mostly in romantic or sexual relationships into my friendships. It feels healthy and enjoyable for me. I think culture has played some part in programming people towards strict emotional categories and relationship algorithms that yield a higher incident of unrequited love. People also not integrating and modulating feelings well together with awareness of one's situation is other-side-of-coin related to this and also a strong contributor. Also, desperation, societal pressure, inability to cope with "romantic aloneness" (percieved to be only fixable through gf/bf) and lack of strong intimacy.