I'm not sure I've ever known and am not certain I will. I can be happy in a moment, but that goes away. Meds help me shrug it off, and I don't tend to dwell on existential shit like this for long, but like...I dunno that I was cut out for this world, yo.
Also this isn't a cry for help and im not particularly sad or upset right now, I was just walking one of the pups and thought to myself "if I can't be happy I'm gonna make sure these fuckers are."
All feelings are temporary, it is silly to have a goal to always feel a specific way. Happiness is a construct that includes a variety of feelings anyway that are still unsatisfactory. That said you can structure your life and mind to make feelings you like happen more often.