For me, it's watching my villagers singing and dancing around the island, and watching gyroids make silly squeaks and noises to songs. I could listen to my gyroids for hours - I even made a room just to hang out with the 'roids and watch them.
What things make you really happy in game that gives that nice boost of serotonin?
What things make you really happy in game that gives that nice boost of serotonin?