Do you ever feel unhappy when you shouldn't be?

It sounds like depression. You don't have to have a reason to have depression. There are many types. Some are situational, like something bad happned and one gets depressed. That usually goes away.
But, some of us just have chemicals out of whack and we get depressed for no reason. Mnay people stay this way all the time, ain't that sad!!
It sounds to me like you have minor depression recurrent. There is major depresssion recurrent, but unless you aren't telling us really how low you get, it sounds like minor. Its simply biological and there is nothing wrong with you, your life, etc. Its just chemicals.
Doctors can help if it gets to the point you cannot or don't want to live like that anymore.
The issues with anti-depressants is that there are many of them and often times if one doesn't work, you have to go through trying many until one does work. Not all doctors are willing to go throught that.
Most also cause sexual dysfunction. So, if you go the doctor route, I would see if they have any new ones that don't intefere with sexual stuff.
But, honestly it really is just the chemicals that are not being made right in your brain. They really don't know why, yet.
Good luck to you.

nope never...

i did once feel happy when i shouldnt have... i have to admit i got away with it but Bashful and Doc were not amused.