Reddit asks the question.  Here are a few answers:

• Anyone and anything at all that’s loved by you. =)
• …pizza with sausage.
• You don’t dread waking up and facing the day.
• … happiness is not a goal in life (“all I want is to be happy”), but a reward for living for something you really care about.
• For me it is an early Saturday morning wake & bake with coffee.
• I think the better question is more along the lines of, “What is contentedness?”

What is happiness?