What do you do that you consider fun? Shouting at the top of your lungs at a football game, bar drinking with friends, betting on horses or gambling at the Casino?
You must really like it; otherwise, why would you be doing it?
Does it make you feel better? Surely, you do when you are carousing with friends. Or when you are heavily engaged in a captivating conversation. You are listening intently, you are offering insights, you are glad that you are there, in that person's company.
When you are having fun, you are smiling, you are happy. Do you have to be with others to have fun? Certainly not! Doing what you enjoy, exploring new territories, playing a game of bowling by yourself can be as much fun as playing against someone else. Shooting pool by yourself can be as engaging playing solo, as it can be with a playing partner. Speaking of pool, taking a swim, doing laps can be fulfilling and make you feel accomplished. That can be fun too.
But do what you love and love what you do and HAVE FUN while you are doing it.