Happenstance is a chance event: something that happens, with nobody planning it. People often say by happenstance to mean by chance, or by accident, though actually it sounds a little more story-like and old-fashioned than plain by chance. You will hear it more in American English than in everyday British talk. It is not a science word. It is a slightly literary way of saying that luck, not a plan, put two things together. A meeting in the street can be happenstance; a whole chain of small accidents can be happenstance too. Do not hear happy inside it and think of a good mood. The old piece happ meant luck or chance, not a smile.