I often find this and while I can't speak for you, IME it's because I don't go out very often so when I do the world is fresh and exciting and different to what I'm used to seeing, so the bit of my brain that is caught up with negative and self-destructive thoughts and feelings is kept busy with "new" sights and sounds. It's essentially an unforced distraction technique. Also, I'm alone at home so it's easier to get away with the aforementioned self-destructive behaviours but it in public, it would be too easy to be stopped so I think that I unconsciously decide there's no point and to save it for when I'm home.