Emotions come from evolved instincts to act. E-motion. Causing motivation to do something. Cooperate with others to earn a living. Motivation to produce off spring. Etc. Laughter is one that is very difficult for psychologist to understand along with art and culture.

In the brain the main circuit is between the lymbic system (a more primitive part of the brain) where emotions come from and the pre frontal cortex (the more modern part of the brain) and the communication between the two.

The brain has to perform a balancing act between the more irrational emotions (lymbic) and the more rational/ logical (frontal lobes). When this balance gets out of whack it causes problems.

Hormones play a very large role. It is not well understood at all and as far as medications it is trial and error. No magic pills although hormone replacement therapy helps many women and many psyche meds can help those of us where things get out of whack. The brain is very little understood by science.