This happened to me alot, but it's a good sign that your Pokemon like you! :D Especially if you give it a Soothe Bell, that helps alot. This also means that Return will be a smart move to use now. ;) Also, Pokemon gain happiness when they evolve! Unlike Ash's Pikachu, Pokemon want to evolve to help they're trainers more, and when they do, they're happy they can help!