I get told this a lot as well. I just don't feel the need to smile to show that I am having a good time or that I'm happy. Usually I'll let it go but lately it's been really annoying me when people say 'smile' to me.
example: Last week I was at a friend's 18th birthday and I was watching a routine dance . My friend next to me told me to smile because I didn't look like I was having fun but it was actually really interesting and I was enjoying it . Maybe ,for a split second , she forgot that I don't really act like normal people but I sure didn't let her comment go. Usually I'll just fake smile but this time I went on a ''mini rant'' about how smiling is not necessary. From now on I think my friends will understand, but others wont.
It's great to see that other people don't feel the need to smile. I feel normal now .