My eldest dog, Glenna, (she's thirteen now), has got to that stage in life when getting up the stairs is becoming difficult for her. It takes her a long time to climb them and from the look on her face it seems each step causes her great pain. However, this is only when I tell her to go upstairs when she doesn't want to. Otherwise she runs up and down them like a puppy.
I've also noticed that she is finding it very painful to "get off the furniture!" when I tell her to. But apparently, it causes her no pain whatsoever to get on the furniture when I'm not in the room.
It's a bit like the intermittent, profound deafness that married men develop as they get older.