The thing is that speaking to yourself is not always a bad thing. Speaking is a pressure valve for our thoughts. If you spend enough time alone, it is dangerous to internalise your thoughts too much. When you internalise your thoughts too much you can have a conversation with yourself to a point where you might develop another personality. This is purely my theory, though, and cannot confirm if that would happen. However, I have spent alot of time alone so, if I am not actively thinking (IE working on something, writing, etc.. basically keeping my brain active), my thoughts linger and turn sour. The only release valve for me is to talk to myself but in a way that does not suggest another personality.
I have a very active brain that is on high speed so even at the moment I wake up, my brain is already at it, usually about something that I have to think about IE something I need to do that day, solving a problem, etc...
So, if you do talk to yourself, make sure you don't pick up a different personality because that's when it gets dangerous.
Even if you are not a person who may usually talk to yourself, it helps when you are trying to pep yourself up IE boost your confidence. Verbalising your thoughts can give them more power because they conscious words, words of choice, and not subconscious fractures of thoughts.
That's all I have to say on the matter.