Confused...?

My boyfriend and I have been together for 8 months and every now and again he tells me he's nervous for the future or he's nervous about making it to a year. And I can't seem to understand why he feels like that. Neither of us have had a relationship this long so I don't understand why he isn't excited. For some reason when he gets extra nervous he'll joke, or sometimes be serious, and be like "we might have to break up". Has anyone else's SO acted like this? What should I do? Why does he feel like this?