Look in the mirror. I mean right now. Are you smiling? I didn't think so.
I have the same problem and I've been working on it. Ever since I was a kid watching Sesame Street, I knew I was supposed to smile. But smiling, if not practiced daily, can seem like kind of a chore.
I recently started thinking about smiles when I joined a Zydeco band. The band I was in had broken up, and I wanted to keep playing, keep my chops up. Zydeco is a unique musical experience for a drummer. Most of the songs are two steps, shuffles, straight beats and a few waltzes thrown in. The endings are cued with a lick from the accordion player. So I was practicing these songs on my practice set and caught sight of myself in the mirror. I noticed that when I play drums, my face looks more like I'm passing a kidney stone than having a good time. And this is Good Time music. So I had to start practicing my smile too.
Just getting the old face muscles to form a smile took some effort. I noticed
it made me look younger, and at the gig, people would smile back when they saw me smiling. I started smiling in the grocery store, when I was at the bank, when I greeted customers in my day job. Then I started trading smiles with strangers on the street. I thought maybe they'd think I was crazy but I was just working some muscles in my face and people started reacting in a positive way. Friends asked me if I'd won the lottery. I started feeling better about my job and my life.
My wife started looking at me suspiciously.