Remember when we were kids? We used to do what we want, and not care too much for what others thought. Imagine if adults could be more like kids? Wouldn't it be fun. I'm not sure what I world would look like - but it would be different.
Once a day, at least, do something fun. It could be singing in the shower, it could be saying hello to a stranger, it could be jumping on the bed… heck, you could even put on wellies and jump in muddy puddles.