1. Your inner 8-year-old brat rears its ugly head
I don't care if you're the most well adjusted adult on the face of the planet, being in the presence of a parent will always bring out the whiny, ungrateful, snot-nosed brat that lies within. What little patience and tolerance you once had flies out the window only to be replaced by deep-seated frustration and a horrid attitude. You're left wondering, "Who is this evil overgrown child and where have they taken my once agreeable adult self?" Then two things are likely to occur: You're instilled with a very real fear of ever having children of your own, and you realize that the post-college therapy didn't work a damn bit.