Thanks Dursleys!, those are the words that Harry Potter should tell the Dursleys according to
Chip Frasier, author of
Philosophy of a Dashboard Saint. The Dursleys have toughen up Harry, with no intention of doing it, making him the strong feeling and courageous teenager that he is now. Without them Harry would probably have been just a regular kid who wouldn't have been up to the tasks that he has been having for the past 5 years.
Psychologically speaking, Harry has undergone such an intense barrage of negativity that it is unlikely that he can go unblemished. Even the most cheerful of us, if we absorb enough verbal abuse, will surely demonstrate a modicum of anxiety! It is only fitting, that when we are continually admonished, that we strike back.
Having said this, perhaps Harry has learned through the years that one can tune out the routine, and if the dream, faith and hope is great enough, can rise above the circumstances to reach that level where the harangue is scarcely heard. 
