The satisfying part of eating?

The first bite... because there's so much buildup and anticipation.

Our brains are hardwired to make eating a pleasurable experience, which is a good thing because otherwise we'd all starve. Asking what feels so good about eating is the same as asking what feels so good about sex or going to the bathroom. The answer is simply because our brains tell us so to make sure that we don't forget to do those things.