There's nothing like starting your day off with an ice cold shower. Besides all the studies on the physical health benefits of cold showers, there are also quite a few mental ones. Sure you'll be training your mental perseverance, but it can also cure depression.
Of course there's the complex science explanation including how due to the increase in production of some chemicals and so on and so forth.
The easier explanation is, if you begin your day with a heavy dosage of torture, everything else will seem much better.
To make sure that your body has a harder time fully adapting to cold showers, every second shower, take a hot one.