Why do I sometimes break into bursts of laughter for no reason?

Ok so sometimes I cry for no reason (and a lot of people do), but also I can burst into laughter at any given moment... for example, I was in a tremendously happy mood the other day just humbly waiting for the bus to arrive and there were a load of pigeons eating bits of bread on the ground and I honestly found it hilarious. I burst into laughter for about 10 minutes... you would think I had just won the lottery or something and everyone kept on looking at me. By the way I do not take drugs at all so that isn't the reason.