Typically we tell other people that we are in a good mood or a bad mood. Moods are generally accompanied by other feelings, such as pain or pleasure. Moods also tend to be transitory.
Emotions often accompany moods, but are more specific. Instead of a good/bad distinction, we have emotions that can flip-flop between being good or bad. Sometimes we don't even know what the hell we are feeling.
But what are moods and emotions anyway, and what causes them? Why do we have moods and emotions?
For example, let's say you're a business executive who has spent a good portion of your life dedicated to your company and rising the ranks. Suddenly and unexpectedly you are let go. As you walk out of the building that was your former workplace, you feel an immense surge of anger, regret, despair, and your mood becomes negative.
This may sound like a silly question, but why would you feel these emotions? Why would you have a bad mood? Why is it that we don't walk around all day happy-as-a-clown with a smile on our faces and a spunk to our step?
Emotions and moods seem out of our control. If something truly bad happens to us, it seems impossible to smile (genuinely). And if something hilarious or great happens to us, it seems impossible to cry out of sadness. All those self-help books claim to be able to show you how to control your own mood and emotions just out of pure "will" - but why should you have to be trained to control these emotions? How likely is it that you can "naturally" become the dictator of your own emotions? You might as well tell me to tell myself to see a pink ocean, or feel sandpaper as a pillow. It's out of my control.
Although emotions and moods are transitory, they still last for a certain amount of time. Something sustains them. From the biological view, the sustainment might be dopamine (or the lack thereof), or perhaps serotonin or adrenaline. But typically we don't say "I'm sad because I have a low amount of dopamine firing in my synapses." We say we're sad because we lost our job, or a pet, or something like that. In other words, we have reasons for feeling the way we do.
But this leads back to the previous issue: why do these issues cause us to feel this way? What are the triggers?
I have some tentative answers:
Since humans are a product of natural selection, and since the mind is (likely, stfu epiphenomenalism) a product of natural selection, and since emotions and moods don't seem to be solely under our conscious control, it would seem that emotions and moods serve a function. Although I can try to limit the influence of my mood or emotions (and thus counteracting the function of them), I have no control over the actual experience of these moods and emotions. I still feel this way and I can't just "stop" feeling this way. It's quite strange actually.
Functionalism and representationalism often get flak for not being able to explain qualia, and although this is a separate issue, it is quite phenomenologically obvious that the anger (a qualitative experience) I felt caused me to punch a hole in the wall. The anger was perhaps caused by me losing my job. It built up, and up, and up up up up UP until I presumably lost control and made a hole in the drywall.
So the self can be seen as a kind of executive decision-maker. The self is influenced by all sorts of things, including emotions and moods. It has no control over these emotions or moods because in fact these are part of the self itself.
The self is not "free", it's just a necessary component for a decision. It's the body's way of enslaving itself.
A human may feel depressed after losing their job because losing their job meant a loss of an ability to survive. The sadness and anger involved in this serve a functional role as an internal motivator - you're feeling depressed, getting a job will restore your ability to survive, having an ability to survive will end this depression, you don't like feeling depressed, so go get a job! Let it be known that the source of the depression is the body itself - so it stands that the body is hurting itself to motivate itself.
Similarly, a person who has a good mood and positive emotions will perform jobs (and thus survive) more efficiently. They will want to continue to survive because this means they will continue to feel good. Thus this is the way the body rewards itself for doing productive things.
Emotions and moods are also usually quite irrational but nevertheless poignant and persuasive, almost in a tyrannical way. People do stupid shit because their emotions got the better of them. They may punch a hole in the drywall simply out of anger - an anger that in the evolutionary past would have allowed them to release their strength against an opponent but in today's day and age makes it nuisance. It nevertheless is a force that causes us to release it - like a spring primed for release.
Since these emotions are generally irrational and primal, and thus not under the influence of the rational self directly, things can go very wrong when someone has a psychological disorder. In mood disorders, this means that people can have bipolar disorder, depression, schizophrenia, or the like. They don't have direct control over what they feel, just like everyone else, but their moods and emotions are going haywire. They require medication and therapy. It stands, then, that the only way to truly change what you are feeling is not by pure "will" but by changing what you encounter on a daily basis (thus emotions and moods are motivators).