You may have noticed that when in a good mood, you'll either relive a pleasant experience, or indulge in a pleasant fantasy about something you hope will happen.
Conversely, when in a bad mood, you'll stew over an unpleasant memory, or will worry about some potential catastrophe.
And either of these moods can strike without any immediate catalyst, i.e., anything good or bad happening in your life.
But is it thinking about fond memories which puts us into a good mood? Or do biologically-induced good moods steer us in the direction of pleasant reveries?
Are bad feelings sparked by a random series of thoughts which then cause us to remember something unpleasant, and then go into a funk? Or are they, too, primarily functions of our neurochemistry?
I suspect it's the latter. When your serotonin and endorphins are going full blast, and you're well rested, and your blood sugar is just right, that predisposes you to travel down neural pathways which stimulate the pleasure centers of our brains.
And, when you're exhausted, you tend to become more paranoid and think negatively.
Certainly, unpleasant experiences can put us into a bad mood. When bad things happen, that makes us feel bad, no question. But when nothing particularly noteworthy has happened recently, our mood ought to be independent of recent experience.
I know I tend to think more pleasant thoughts right after having exercised (when the endorphins are circulating), or right after having eaten. Or when I'm well rested.
Think of how alcohol can put us into a good mood, or cause us to think silly thoughts. My guess is that our normal biorhythms can have similar, but subtler effects.