Relationships

When we get married, in theory, we marry our best friend.
Today, as my best friend and I were driving down the highway and the toddler, my new BFF  was singing in the back seat, a song which was easily recognizable as the alphabet song, as sung by a drunk with slurred speech, I mean, a toddler, I was musing on how good things were.  How well spouse and I get on, how happy and enthusiastic our children are.  
But then, no sooner had we stopped the van and started to get out and I said something about my plans for Sunday (a jewelry party a friend is hosting, and yes, I plan to attend if for no other reason than I like looking at sparkly shiny things and she puts on a good spread) and he started in on how busy he was between then and now and getting huffy with me for making plans for the weekend.

So I ask you, what is it about relationships that make us go from best of friends to me thinking of driving off and letting him walk home?  Is it that we're much too familiar with the other and have history attached to every  action, or is it just that we get comfortable with the other person being our best friend and don't understand when suddenly they're acting more like someone else?