@javybar sir...
Off the top of my head, there are 2 reasons why it is important to have fun.

1. Having fun is a key part of re-enforcing habits.
   From a biological, evolutionary perspective, people want do things because it is fun. When you feel like something is fun, it is because you are satisfying your fundamental human needs. Partying is fun because you are bonding with other people (if it is a good party) and bonding with people is essential to survival. Sex is fun (assuming the two parties are attracted to each other), because reproducing is fundamental to survival and again your bonding with someone. Work can be fun, if it is work that truly satisfies your need for accomplishment, self-esteem and you believe your truly doing something worthwhile. So if you want to re-inforce a positive habit, have fun with it. When you are having fun, you are releasing neurotransmitters called dopamine and serotonin. For this same reason, this is why one can become addicted to vices and negative habits, such as drugs, excessive tv, video games and porn. So if you want to reach your definition of success, have fun with the positive habits that will get you there as much as possible.

2. People want to be around people who are having fun
   Why do you think for every advertisement you see, the person using the product or doing the action, has a big smile on their face and looks like they are having the time of their lives. Having fun attracts people, and when you bring people into your fun, you have more fun because you are bonding with them.

Thanks sir...