If this implies you are not yet an adult or a young adult, it may also explain why you do not understand it (no offense meant just pointing out it may be a lack of perspective).

I suppose I mean "serious" as in my demeanor and general attitude is mostly tied up in those things like responsibilities and the deadlines and conversing about business, world events, catching up, etc instead of all the free time, lack of responsibility, and ensuing shenanigan's that comes with youth [my youth]. As you age you slowly accrue more and more "responsibility" such that it's difficult to even get in the right headspace, company, and context that would even present a side-splitting laugh situation. It's not that I was overly stoic. I laughed, hard even, but not where I couldn't breath or I was literally rolling on the floor.

It's not that I somehow lost the ability but it's like if you stop working out because the new job is too busy or you are spending all your free time caring for a household/relative. Your health probably doesn't decline a ton in the initial months or even year. But fast forward 10 years and you find that not only did you not work out, but you also let your diet slip, and maybe accrued some weight. Sure you can visit the gym, you didn't forget how to work out, but you lost the habit and you have to work a bit harder to get back what you once had.