Maybe because our brain needs more resources to compute every other thing and it fails? When you're nervous or stressed, there are a lot of biochemical processes happening throughout your body like the production of cortisol. If we are nervous in a situation, we are trying to think of every possible thing that can go wrong, we are trying to see how the person we are talking to expressed himself including judging their expressions, tone of their voice, etc. We do not really care about these as much when we aren't nervous and take things in a chill manner.