Ego has nothing to do with the capitalization of the pronoun I. Printing and handwriting have everything to do with it. In Middle English the first person was ich--with a lower-case i. When this was shortened to i, manuscript writers and printers found it often got lost or attached to a neighboring word. So the reason for the capital I is simply to avoid confusion and error. Of course, some writers refuse to be bound by this convention. Two of our favorites, the poet e.e. ***mings and Don Marquis, author of archy and mehitabel, both favored the lower-case i" (Morris Dictionary of Word and Phrase Origins 303).