That is a good question. I think as the others have said there a lots of different factors that all bear some weight on whether you tear or not. Some you can try to influence (birthing position, technique - for want of a better word - when crowning etc.), and some that are just luck of the draw (shape and position of the baby, shape and position of your 'equipment' etc.)

I think I am a bit with you, it is just something that happens sometimes and not others and you roll with what you are given. When I was doing prenatal aquarobics there was some discussion on preparation for labour to try and avoid tearing and our instructor who is a mother and spends a lot of time with pregnant women said much the same, if it happens it happens and you deal with it and then you get on with it. I mean tearing and the recovery is not fantastic but it is part and parcel of what I signed up for!

On not tearing being an achievement, you know I had never thought about it that way but now you mention it when I hear or read other peoples birth stories I do get a sense of either a) achievement OR b) just pretty darn happy. I guess sometimes it is hard to tell the difference (especially in the written word) between the former and the latter!