If we get back to our Christian example, we can consider our body's as temples of the Lord. We use our temples to spread the word. That's the ultimate goal for Christians, to spread the word and get to Heaven. So, in this way, it's our duty, it's our responsibility to take care of our bodies, which are essentially on loan from God.
So, if you take care of your body, you're naturally going to feel better. You are naturally going to be happier. I mean, it's in the science too. The more you exercise, the more endorphins are released into your bloodstream. These endorphins are your bodies natural morphine. They make you feel good.
Ever hear of runners high? People, after they run, experience a feeling of euphoria. This is why they get addicted to running. That is why I run. It is to experience this natural euphoria. The reason for it is because of the release of endorphins.
It's God's way of thanking us for taking care of His body; His temple.