Because in order to do anything that's fun, like hanging out with friends,
learning math, going to the beach, reading a book, playing with a dog,
going for a bike ride, going to school, and like a whole bunch of other
things like those, you like have to be living, or else like none of it works.
The correct answer is this: AN INCREASED LEVEL OF DOPAMINE THAT DOES NOT REDUCE BRAIN SIGNALS.
Dopamine is a neurotransmitter, a compound that is released by the brain and it is associated with attention, addiction, motivation etc. Getting involved in physical exercise increases the amount of dopamine in one's body in an healthy way.