(The list of "31 Days of Reasons" is not in any particular order)
Being active - and healthy in general - allows your body to better cope with stress on a chemical (yay to endorphins!) and mental level.
From what I understand (thanks to Dr. Google and Dr. WebMD):
stress = higher level of cortisol
higher levels of cortisol = getting sick
physical activity = less cortisol
physical activity = more endorphins
less cortisol + more endorphins = happier you
I'm on this healthy journey to improve my physical and mental health.