Why does being in a bad mood decrease motivation to do things to feel good?

It's about dopamine and serotonin (and many others) but that's only the mechanism.

The deeper reason is that depression is an evolved physiological reflex for self-protection, and it is seen in organisms as primitive as lizards and lobsters. If an animal existing in a social hierarchy (like humans) suffers some kind of defeat, rejection or humiliation, or finds themselves stuck or trapped, further activity often makes things worse.

An example is a defeated lion - he can retreat and literally lick his wounds, but continuing to fight could be fatal. Better to come back a few days or a week later and try again - in the meantime, everything shuts down: motivation, interest, libido - they are all switched off.

Problem is with humans is that our problems are more abstract and enduring which means the depression reflex can stay switched on long past its usefulness, and it even becomes the problem itself - exacerbating the original defeat. So humans need to develop tricks to break out of it - like forcing oneself out of the house through will power, exercising, or taking medication if it's severe, which essentially tricks the body that the depression reflex has been turned off again.

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