Do you go for the challenge or the easy?

ive noticed people tend to go for the challenging people. is that true. is it just our natural instict to go for people that are harder to get? is it because if someone is to easy, they are just too available to anybody so ppl dont feel like they accomplished something or something? if youre too nice and easy, are you lowering yourself and getting people too look down on you? is being a little bitchy a neccessity? just curious on how ppls mind work :) thanks