Trust me, if you managed to make me laugh, then told me something positive that happened in your life - I'd be pleased for you.It could also be that there's just something about you that doesn't inspire a lot of empathy.
I wouldn't stop smiling. You have to admit, for someone to stop smiling once they're told positive news, is very strange.
It's not like I was looking for empathy because something negative had happened, that could raise questions in their head, like for example: "My wife has just left me!"