You cannot speak of grave things while feeling aloof. Speaking of grave things makes you grave.
No matter what mood you are in, if you start talking about serious subjects, you will become serious. Don't get me wrong. If you're in a jocular mood, you can make jokes about serious subjects. You can use those subjects as fodder for your humor. But if you actually sit down and think about those subjects, if you sincerely discuss a serious subject, it will make you serious.
It is an amazing power. You can use that power on people, too. If you have a serious subject you wish to express, you can suck people in with an amusing introduction, then strike them with a serious subject. Once they've started reading, they will keep going. Once you slide into a serious subject, they will become serious, think seriously, but not be disgusted by it, nor will they push away with humor. They will be yours.
Of course, with great power comes great responsibility. The same tool that can make people care about doing good deeds can also be used to scar them. A troll could, for example, start showing you fluffy kittens, making you happy and relaxed, then show you a picture of roadkill. It's definitely a dick move.
There is nothing I am saying about "should". I'm not really talking about right or wrong here. I'm just saying that this is how we work. You think about the grave, you become grave.