Why does my boyfriend sing randomly in public?

We have been together about 6 months but lately I've noticed that whenever he is in a good mood he will randomly start singing loudly even if we are out in public. I find that the problem worsens when he gets drunk but when he is sober it is still bad. I find it really embarrassing and he doesn't even have a very good voice. People stare in the street as if to say why is this loon singing and i always want the ground to swallow me up. I really love him and this is his only imperfection I just want to know why he does this and if anyone else does this? Is it because he just loves music or is there a mental health problem that needs to be addressed?