Is it biological, related to survival? Do the chemicals that are released give off pleasure signals that let the brain know it's safe and secure.

I'm very curious about this, I think most mental illnesses can be pinpointed back to this natural phenomena, a natural propensity to feel happiness instead of sadness.

Except those who feel depressive, bipolar, anxious, panicky.. exacerbate these feelings and put all of their focus into trying to make themselves happy when it's not possible because that's not in our control. Just signals used to help one figure out what's going on in their environment.

Just my theory, what are your thoughts?