Why do stan, kyle, kenny and butters care about cartman so much, even though he's a total douchebag?

He's a total dbag, yet, you always see the others care about him so much. Like in the episode where cartman wants to be a nascar driver when he grows up, but he says he's not poor and stupid enough to be one.

Why do stan, kyle, kenny and butters care about cartman so much, even though he's a total douchebag?
Post Opinion