This is an answer to a certain misanthropic acquaintance's question.
When you're asked this, what immediately springs to mind is things that make you happy: To be held at night, to have made something that didn't exist before, to walk in a field of tall grass with the sun on your back and the wind in your hair, to give 24 million dollars to ACLU, to curl up with a good book and a barrel of cheese puffs while the rain and the night crashes against your windows, to crush your enemies and see them driven before you, to dream of flying, to find a lost or unexpected treasure, that sort of thing.
If pressed, we'll be forced to admit explaining what makes you happy does fuck all to explain what it feels like. To someone without those shared experiences or associations, your analogies don't mean anything. You'll mumble something about if they have to ask they'll never know or some things are just so precious and fragile they fall apart if you try to examine them and give a shrug, with your eyes downcast. If you're an intellectual coward.
Me, I've had to train my whole life to try and see things from other points of view. To examine things without a frame of reference. Yes, happiness is a volatile, elusive evanescent feeling. Any self-help guide of substance will tell you to pursue it is meaningless; what we want is to feel contentment, importance, comfort, trust, welcome, understood, well. Happiness happens when we're busy giving substance to the world around us. It's not something you should or need to dwell on.
But what it feels like is this: A gentle calming warmth rising steady and level in the upper part of my chest, tugging at the corners of my eyes. It gives me associations of possibility and the end of loneliness. It makes it impossible to remember what it feels like every time I say something and stare blankly, ignore it completely or read it in a way I didn't think of. Maybe the happiest I've ever been in my life was when i first saw the climax of Neon Genesis Evangelion where Shinji asks "what are you?" and Rei says "I am the hope that people can understand each other."
I don't know if that makes any sense.