What could it be with my cousin?

Question

I have a cousin who loves to smile and laugh, all day everyday. But it's getting to be a bit disturbing because you could be having an everyday, normal convo with her, or even when you talk about something sad, and she'll smile and bust out laughing when nothing's funny. I love happy people, and I love to smile myself, but her laughing is a bit disturbing. Could this be just a stage?