That unrequited love

Most of us must've had that one unexpected unrequited love which tugged at our hearts more than any other, for obvious reasons. I've always wondered what it is that made this unrequited love so strong. Was it the human greed for returns? Was it the fear of failure? Because, wasn't love supposed to be selfless?
I have one such unrequited love in my life. I've found myself putting a hand on my chest, trying to calm that beating machine or maybe just trying to understand it. Of course I could've always blamed my hormones; but this time it was different. Every time I tried to run in the opposite direction, the reality smashed in my face. I couldn't escape this. I could just accept it and reconcile with it.
Could I is the question, isn't it? Well I think I'll let you decide.