As quoted from Bill Watterson:

"Susie is earnest, serious and smart. . . . I suspect that Calvin has a mild crush on her that he expresses by trying to annoy her, but Susie is a bit unnerved and put off by Calvin's weirdness. This encourages Calvin to be even weirder, so it's a good dynamic. Neither of them quite understands what's going on, which is probably true of most relationships."

As quoted from Me:

Susie is Calvins sworn enemy. She is the reason he started the organization G.R.O.S.S. (Get Rid Of Slimy Girls(Susie)). He is always pelting her with snowballs but he gets his just desert. When Christmas rolls around though its a whole different story because Calvin wouldn't want to jeopardize his chances of recieving everything on his list from Santa. Calvin, in my opinion, has a crush on Susie, and the evidence is in the way he constantly torments her and when he truly portrays his feelings on Valentine's Day.