How do you respond when someone asks you "why aren't you smiling?" why do people expect you to smile 24/7?

I got a brother who says "why do aren't you smiling" when we were at the venue for an event. I'm thinking it's stupid bec. just because I don't smile the whole entire time like weirdo. I mean do people really expect you to smile the whole time? I don't like the way my bro says it. I told him "i'm not sad i'm just not smiling the whole time". My bro thinks i'm mean because I don't call him to checkup on him on a regular basis even though he's grown. Let's say i'm in my early 20s and I got no gf (never had one nor have I dated) and I got 0 friends GEE I wonder why i'm not smiling more often!!! I did smile at the event but not the whole time.