One of our kitties LOVES to have her gums massaged! Interestingly, she is the only one of our six that has a problem with plaque accumulation, though I have no idea if there's a relationship. But it does mean she loves having her teeth brushed, so that works.
I think we were at about 3 years when this happened.... so I can imagine how shocked you were!
Zoey's learning to love being loved!