Why do breaks up hurt so bad even when we know theyll get better we still get upset?

ive been dating since i was 16 and i never got dumped by any one until i turned 20 and within that year ive been broken up with twice, i was always upset when i broke up with people because i felt bad but never had pain like this. i just dont get it like why do we get so attached and why does it hurt so bad? like im young ive heard millions of times and even told people tons of times. you can do better youll find some one else and i know but right now it hurts and i just dont want to feel this way any more. its lame. so lame.