I've talked to myself as long as I can remember. If it occurs to me to do so, I sing or recite poetry. (Quite often when washing-up.) OH has got used to it and quite likes to hear it now (finally brainwashed him!).
Thanks to Buenchico, I now have a reason for doing it! My standard reply for many, many years to people who broached the subject has been 'It's hard to find anyone else of an equal intellectual level with whom to converse.' Accompanied by a smile, it usually raises a laugh. :0)