So my bf says he can't be "himself" when I'm around.. what's that about?

We’ve been dating for 3 years and living together for 2. How can he not be “himself” when i’m around if i’m around all the time?! He went to a bonfire tonight and decided not to take me using this as an excuse. I must confess I’m a bit offended by this. Is this something I should discuss with him when he comes home?