What causes someone to fall in love? What's that one magical trait someone has that causes another person to develop feelings that should only be reserved for the most special person/people they'll ever have?

Fuck if I know, but I'll keep analyzing it and trying to figure it out until I either succeed or die trying.

I've been in love 3 times. A deep, overwhelming love that leads to irrational thought and in my case ultimately heartbreak. Of those 3 times, only once did it actually make sense. A mutual and instantaneous love that was natural and present from the start. It also spoiled me in thinking that it would always be that easy, and more importantly that mutual. The other 2 times were people I knew deep down I could never truly "have", whatever that means. I'm smart enough to know you can't make anyone feel anything, especially something as intense and important as love. That knowledge wasn't enough to keep me from trying though. 

Alternately, I can think of 2 people who were unquestionably in love with me and I didn't feel the same. 

So why would anyone reject people who are giving their heart and soul to you yet yearn for those you know weren't meant to be?

Humans are a fucked up species. Sorry folks, that's all I got (for now).