Maybe you noticed one of them was abusive to the other, Whether it be physically or emotionally. That's what happened with me at least.
My parents divorced but for some reason I don't care - it doesn't make me sad or anything. Why?
Same thing happened with me and my doctor said it is normal for some people not to care about what happens to others.
Perhaps it doesn't make you sad because you know deep down that they will be happier apart. Or perhaps your are just coping with it that way. Either way there is no reason why you should worry about the way you feel.
Maybe because you are a happy person and just don't care for those kind of stressing matters! And yes you shouldn't get worried because in all the ways its not gonna help you! So just stay like this!