Enthusiasm helps dictates your view on the world. Based on how you were raised, you might just naturally radiate enthusiasm. An enthusiastic person can also be seen as a person that knows how to enjoy life.
An enthusiastic person is always welcome. They'll lift their team's spirit, motivate their peers and generally make the world a happier place. Enthusiasm is pretty much infectious as being in a group with enthusiastic people will positively influence you as well.
But sometimes it's really hard to feel enthusiastic. You might want to distance yourself from reality, build a cocoon of safety around yourself and avoid human contact. It doesn't matter who you are, you will encounter setbacks during your life. Some might be small like a minor disagreement whilst others life changing. Career wise you might get demoted or even fired. You might be suffering from a bad breakup. No matter what, life goes on. Sometimes just going for a walk is a great way to clear your head. And I'm not talking about around the block, but just going somewhere without a set goal. Eventually you'll forget your problems no matter what they were. If that doesn't work, you just haven't walked enough. In the absolute worst case scenario, your mental pain will eventually be overtaken by a physical one.
Remember that fun doesn't need a reason. Focusing on how others might perceive you will limit your options. You'll feel small and insignificant. Christianity literally states that all human are born sinners which isn't exactly the best way to motivate a person.
Enthusiasm is hard to gain and easily lost. Focus on what you're grateful for and find people who actually care for your well-being.
If you are that super enthusiastic person, it is 100% more likely for someone to remember you. You'll transform the mental image that they would have created of you from an ordinary, standard, boring, unimportant, insignificant person into one of person you want to be around.
Life is full of these mystical circles of positivity and negativity. Just as the best motivation for a student is an enthusiastic teacher and the best motivation for a teacher are enthusiastic students, unenthusiastic students will make a teacher want to give up on a class which will just further worsen the students' attitude towards learning. As I have already mentioned, humans are social creatures. If we radiate positive energy, others will feel better in turn making us feel better ourselves.
In life, everything has a reason, but that doesn't mean that you should focus on that reason at all times. Sometimes it's better to just go with the flow and have fun.