Exercise makes you happy!
You know how you feel so good after exercising? There's a reason for this.
Just as you get more energy exercising you also get happier! You will feel good and if you don't already appreciate all the good things in life then you might start.
When you exercise your body gets rid of stress and you don't feel as tense as before exercising. Someone who exercise regularly are less prone to get depressed than someone who doesn't.
Add some chili to your food and your endorphin release with be even higher and you will feel happier. That of course only apply if you like spicy food, for me I'll do some extra exercise instead.
But it's not only the outside you need to exercise, the inside is just as important if not more. The better we feel about our selves the better atmosphere we create around us. Making other people happy as well. And if you are enthusiastic about exercising you might pass it on to someone who's not active and change their life.
Spread happiness around you, start exercising!
I think biology affects age but biology alone here is limited and even short-sighted. If we compare the lifestyles and mindsets of older people, its amazing that longevity, resilience and sources of motivation can distinguish among individuals who sit lifeless and depressed in nursing homes vs. mountain climbers.