why I will never love my father like a son should

 it is easy some days to tell yourself you are happy, but sometimes no matter how hard you try that is not, so very easy. want to know something that when I think about it still hurts me to this day? when I was about idk 13 I got a letter from my father, and well me, and my father never been close, I mean I knew him, but he was never in my life besides the few pop ups, so this was rare, but anyhow the letter was your basic blah blah sorry I could not be there letter, but then there was something that stuck out, and I will never forget, he wrote "although I got your mother pregnant with you I will never be your dad, nor will I try to be" and tell this day that bothers me, and I don't know why, but no matter how much we talk now, because we do talk on facebook I can never forget that, and I can not think of him like a son should think of a father, but it seems weird, because it was not like he was ever in my life, so why does it bother me, so much? should I be sad over this all these years later? should I really let these few words keep me from thinking of my father as a father? I mean I know that we should forgive people, and I do forgive him, but why does it bother me when it passes my thoughts?