Exactly. I'm not a psychologist or biologist but from what I read, while we are alive, our bodies tend towards homeostasis, a chemically balanced state. Simply, you cannot be in a high-dopamine "happy" state all the time without going back to a neutral or down state afterwards. Even without drugs, highs are usually followed by lows. Not trying to be happy at all times but accepting that sometimes, there are hardships, will help you having a fullfilled and content life.
Anyway, that homeostasis does not mean that everyone is equally happy/unhappy on average. On the one side there are people with depression and on the other side people whose lives have many happy moments. What helps for me is connecting with friends and family, going into nature and seeing animals, doing exercise and having off-screen time, mindfullness and generally having some work-life balance. Which I admit not everyone can afford and I am privileged to have. Doing or having those things helps me having a happier and more fullfilled life, but sometimes life still sucks, there is no happy-all-the-time.
Sorry to hear that. Though a broken pain must not necessarily mean severe pain. It depends on which bone you break, where and how. You got unlucky there. I had a couple of broken bones, my big toe, my hand and part of my hip (not the joint, but the Illium, which has a purely protective function), and in all those cases the pain was managable without painkillers, as long as I didn't move the injured body part at least. It was always a closed wound and there was a crack in the xray but the bone had not moved much out of place and never needed an operation. Also adrenaline played a role, I broke my hand at a Judo competition and at first just thought I just bruised, but the paramedid noticed that it moved where it shouldn't.
With all my traumatic injuries I was consoled by the fact that soon I will arrive in the hospital, will be cared for and the pain will stop.
The worst pains I had were a severe tooth ache and strong migrain attacks. It was worsened by the fact that I didn't know when the pain will end.