The Doctor sometimes does meet his previous incarnations. He doesn't do so very often, however, since it's contrary to the First Law of Time. In the classic series on TV, it happened only thrice: in "The Three Doctors", "The Five Doctors" & "The Two Doctors". In each of those cases, the meeting between incarnations was the result of interference -- either official or unofficial -- by the Time Lords on Gallifrey. In the revived series, with the Time Lords out of the picture, it has happened twice, once as the result of a serious accident, in "Time Crash", and the second time as a result of the Moment's interference in "The Day of the Doctor". There is also the special case of the Doctor exchanging words with the War Doctor within his timestream in "The Name of the Doctor".