Why does the pain of love hurt so much, so much more then the pain of the blade?

With just one touch, I fell in love with my best friend, with all the pain floating around in the air and in the pit of my stomach, I just could not take it. I mean, how could this have happend? Was it the why she talked or the way she walked? Maybe the why she cares, it could even be the sparkle in her eyes? But still... Why? She is my best friend not my lover. Either why you know you are in troble when you can't even be around your best friend who has been with you by your side though everything without getting light headed and butterflies in the pit of your stomach.