The Bible says to forgive and forget, and in my life I have always felt much better forgiving and forgetting than walking around with that weight on my shoulders of hurting and not forgiving.  Harry feels the same way, and at some point Severus has talked to him about this behind scenes since Severus reads the Bible.  I have always thought that people hold back their forgiveness feeling as if they are getting revenge or something else, but really they are hurting themeselves.  Honestly, it feels so much better to forgive and be done with it.  Too many times in stories I feel like characters never do forgive each other.  In my own personal opinion, I don't believe it is ever too soon to forgive.

That being said, just because you forgive doesn't mean you have to be friends with somebody who will hurt you, but Harry was so hurt by not being friends with Ron, that he wanted another chance.  Also, Ron is an ass in a lot of stories so people think he's been that way his whole life in all stories.  For the sake of mine I'm going to say he's only had a couple of those moments, and it's not an every year sort of thing.  For Harry though he's used to other people being really rude every year to him whenever things go wrong.

Too many times people refuse to forgive, and my faith in the Bible tells us to forgive and forget.  It's not easy to forgive, but in my life I've found it so much better to forgive and not carry the weight on my shoulders and the bad feeling that accompanies it by holding grudges and refusing to forgive.  Now, that being said, lessons should also be learned, and just because you forgive somebody, doesn't mean you have to be friends with them, it just means you forget it and don't bring it up again.

Since I wrote in that Severus reads the Bible, I want readers to assume that at some point he speaks to Harry about some of the things contained within, one of them being forgiveness.