Why should a man find it difficult to tell his woman friend about his new girlfriend?

He knows that his friend is not interested in him-on the other hand she always teases about his roving eyes and many girlfriends and encourages him to get a gir friend-and they stay good friends even for a very short period.But one fine day he starts distancing himself from her 'coz she pointed out his short tempered nature and stop calling her.for over 2 weeks doesn't contact her or answer her few messages.But answers with a 'YES" text message to her question if they r still friends.Her friends feel that he may have a new gf.but WHY he should find it difficult to tell her when he very well know that she was never interested in him that way..??