Why do alcoholics talk to themselves ?

My dad drinks very regularly and pretty heavily every day due to depression and I’m just curious if anyone has the experience with alcoholics where they will have full conversations with themselves but as if they were talking to someone else ? Idk if this is how he processes situations but it’s to the point that I question his mental stability. He has conversations as if he’s talking to someone specific (he often sounds angry) but he is just talking to himself.