Good moods are caused by four brain chemicals : endorphin, dopamine, oxytocin and serotonin. It would be nice if they just flowed all the time. But they were NOT designed to do that. Your happy chemicals were meant to do a job. They turn off when the job is done, which makes them ready to turn on when you need them again. Our natural ups and downs are easier to accept when you know the job your happy chemicals are doing. Here's a crash course in your neurochemical operating system.
Endorphins evolved to mask pain. If you're escaping a predator, neurochemical euphoria promotes survival by mask pain until you're in a safe place. But the rest of the time, masking pain does not promote survival. You would run on a broken leg and touch hot stoves if your endorphins flowed all the time. You would not be better off because pain holds survival information. You did not evolve to be on an endorphin high all the time.
Dopamine sparks you when you're near an important reward. If you were climbing a tree for a juicy piece of fruit, dopamine triggers the boost that gets you up the last few branches. If your dopamine flowed all the time, you would invest energy in everything, rewarding or not. Your reserve tank of energy is available for things that matter because your dopamine only turns on for things that are important. You did not evolve to be on a dopamine high at every moment.
Oxytocin stimulates the feeling we humans experience as trust. If it flowed all the time, you'd trust people you shouldn't oughta trust. Orgasm and giving birth trigger oxytocin, as does a massage. All mammals release oxytocin when they're born, triggering attachment to their mother. More is stimulated when a mother mammal licks or holds her child. Mammals transfer their attachment from a mother to a herd or pack or troop, and these social alliances also trigger oxytocin. The bigger a mammal's brain, the pickier they are about which social alliances to trust. You would not be better off if your oxtyocin surged all the time.
Serotonin flows in an amoeba when it can safely to feed. Humans have more serotonin in the stomach than the brain because our ancestors fed when it was safe. Mammals often have their food stolen by more dominant mammals. It's safe to eat when you can dominate, and that's when serotonin rises. Feeling on top boosts serotonin, as challenging as the idea is to modern sensibilities. If your serotonin were high all the time, you'd constantly one-up others and make yourself a big pain in the neck.
Happy chemicals evolved to alert us to survival-relevant information around us. If you surged with happy chemicals all the time they'd have no power to alert you to important information.
Yet it's natural to desire more happy chemicals and to do everything possible to stimulate them. They evolved to motivate us to go toward things that promote survival Natural selection created a brain that motivates you to promote your survival with happy chemicals. A brain that was happy all the time would not need to take survival action to feel good. Survival prospects would fall. Natural selection would weed out a brain that was happy all the time!
My book Meet Your Happy Chemicals shows how you can re-wire your brain to have more ups and fewer downs. To go beyond that, my new book Beyond Cynical: Transcend Your Mammalian Negativity explains why this brain we've inherited tends to go negative. You can PARE your negativity by building your sense of Personal Agency, and Realistic Expectations. But we do well to remember that our happy chemicals are not there to surge all the time. They are there to motivate us to do things that promote our survival.