Don’t take your seriousness too seriously.
It’s okay to have a straight face. It’s perfectly permissible to think deeply. And it won’t cost you your joy if you need to set aside some time to be focused and intense about the things that are important to you.
Sometimes, being serious and not treating your seriousness as a sin is the best way to be light-hearted.
The opposite of happiness is not seriousness, but self-rejection―the inability to be at peace with who you are, however you are; serious or otherwise.