marriage relationships

Why is it that we treat our spouse worse than we would a stranger? We would never snap at someone else for eating too loud, being a little late, or whatever else little pet peeve you can think of. We give others so much grace and none to those closest to us.
I heard a story of someone who was talking about something I talk about a lot and it was like someone slapped me across the face with it. I don't remember the context or even where I heard it but I'm going to paraphrase. This man was telling an acquaintance/friend about how much he was learning about God's grace. About how God doesn't kick us when we're down, how God's not this person who only gives us so many chances and that's it, how he loves us even at our worst, etc etc. When he was done, the friend said that it must be wonderful to be your wife and children. How could you learn so much about God's grace and not therefore, be that much more grace-full to those around you? The man stopped and realized all these realizations he'd had, hadn't transferred into his home. He saw how he was supposed to give more grace to the world around him and even to himself, but not to his own family. He said it changed his life.
As Paul said, I am the chief of sinners. I am so ungraceful to CJ. I expect him to always be courteous, even when he's tired and stressed and thoughtful of my feelings at all times, in all situations. When he fails I have very little grace. However, I expect him to be graceful to me. When I'm tired, I get super crabby. Sometimes I just get in a mood and there's no snapping me out of it. I expect him to deal with it and know that it's just because I'm tired, moody, emotional, or whatever else excuse I give. In other words, he needs to give me grace. Most of the time he's pretty good about it, especially the moodiness. He's learned it's just me and I'll snap out of it sometime soon. (just that much more quickly if he shows me the grace I am so obviously needing) There are some situations that push his buttons that he falls short of showing me the grace I need but who doesn't? I certainly do that all the time.
This issue has been brought to the forefront of my mind recently by a movie I went to go see with CJ. It's called Fireproof. If you haven't seen it yet, go. It's wonderful. The most wonderful thing about it wasn't the acting, although that was better than the previous movies, or even that it was a "Christian" movie, all the "Christian" stuff could have been taken away and this part would have still been left. The best part was that it reminded us to appreciate our spouse and treat them like they are they are worthy of love and grace. To hold our tongue instead of spouting off that little jab, to study our spouse again, their likes and dislikes, what makes them tick. Basically to do the things we used to that made them fall in love with us in the first place. How many of our spouses would have fallen in love with us if we treated them then like we do now? After CJ and I went to see Fireproof, we had one of the best dates we've had in years. Probably one of the best since we've been married! It wasn't about where we went or what we did (we went shopping for Dani and went to Carlos O'Kellys) but how we treated each other.
I know our relationships change after marriage, it's inevitable. There is a study that has shown the intensity of feelings can physically not be maintained chemically in our brain. That intense "I must be with them every second of every day" feeling. To try to hold on to those feelings is not realistic at all. But what we can and should but don't keep up is the basic courtesy, the deference to each others feelings, the unselfishness, and outward expressions of love on a daily basis. How many marriages might have been saved if these weren't done away with after the "honeymoon" phase ended?