I think one of the best things is being able to get my frustrations out in a very creative way. If I'm angry at someone then I can kill them off in a hundred different ways and they will never have to even know. If it's someone I really can't stand, then I can make them a villain in a novel. After all, the villains always lose!
On the other side, if I want to convey a wonderful or thoughtful thing a friend did for me, I can write that into one of my books. I have some of my characters based on people who have been special to me over the years. I would like my readers to get to know them as I have.