Once I have a companion at 10k, I have never seen it dip below that. I suppose it is possible that if you go over 10k it is still there and only displays the 10k, so I might actually be losing affection but it still be remaining over 10k. I always thought that at 10k you just became so attached to each other that you could hit each other in the head with a baseball bat and not care, LOL.