Gotta say, the extreme negativity expressed by the OP gives me a reason to wonder why he/she posted at all.
If the response to everything is “why should I be happy…what’s the point of anything…why do I need to be positive?” one has to wonder if perhaps the OP isn’t doing a research project or is otherwise bored.
But, I’ll offer up an answer: You’re here, and you exist. Why not be happy?
If you can’t understand that simplest of ideas, go see someone who can help you understand why.