Seriousness is not the polar opposite of fun.
Without the other, neither can be more meaningful. Both are essential, and I believe we may have coined them incorrectly, or that we have become clouded and believe that either is bad.
Humour is beautiful and necessary because it reveals who we truly are and forces us to dive into our vulnerabilities and become the object of the joke, allowing all of our guards to come down. Fun times with people we care about are what make memories worth revisiting and solidify strong bonds.
It may not be the most accurate definition, but for me, being serious means having self-control, discipline and sincerity, as well as being honest and forthright in one's dealings with others and with themselves. All of those characteristics can still be found in people who enjoy themselves, making it important to be both.
The key difference, I believe, is in context, flexibility, and awareness.
Context: When is the reserved person or the fun person required?
Flexibility: Can you switch if the situation calls for it?
Awareness: Are your seriousness/fun at the expense of others?
The task now is to strike a balance; this is a solo journey!