Question Regarding Migration

Hello guys sorry this might be an incredibly basic question, but I have been reading a lot about geese and can find answers as to why they migrate South for winter, which as with most birds seems fairly self explanatory. But I was wondering if anybody could explain why they migrate north again in the summers? I ask really in relation to all birds. Are the summers too hot? Is it a food issue? Any help very much appreciated. Kind regards, Hugo.