Brains are muscles - if you're not constantly exercising, using, and stretching them, then they can easily atrophy.
Chess is excellent brain exercise, even if you aren't playing the current champion. Playing anyone at a higher level - or even the same level - as you are can really help you improve as long as you're willing to learn from your mistakes. In every game you lose, it's important to remember the following points:
1) Don't take losses as failures. Winning doesn't encourage improvement, while losing can be the motivation and lesson you need to take your game to the next level.
2) Make a mental (or written) note of the moves that ultimately resulted in your loss or draw. This can help you avoid the same pitfalls in the future as well as figure out creative new ways around them that can only bring improvement to your whole game, as it inspires a new way of thinking rather than memorization of a preset of moves.
3) Similarly, appreciate good moves from your opponents. It's not fun to have someone pull one over on you, but if it's done elegantly and executed well, you should probably keep it in mind. It'll feel twice as nice when you get to use it on someone else!
4) Take advice wherever you can get it. If someone -- even a child -- offers you well-meant advice, don't just snub it. A fresh pair of eyes can often see things that experienced eyes have overlooked.
5) Keep playing! No one enjoys losing. It's not in human nature to have fun in defeat. However, remind yourself that there are many benefits in your lost match, and not only that, chess is something you should always love, so be happy doing it!
Don't forget to exercise your brain, have fun, and learn wherever you can!