You are so right - mine are not crazy about having their mouths or teeth examined ( except Sami & Max who couldn't care less)- Bella whimpers but doesn't move but the funny one is Graycie,she is such good girl and should be completely used to it and she's doesn't struggle or resist until she thinks medicine is coming- then she's terrified for some odd reason...Absolutely this! Neither one of my cats will allow anything to do with their teeth and they are 10 and 16 1/2. I started both of my dogs (one is 4, and one is 6 months) from day one and it REALLY pays off to have them used to having their mouths handled from babyhood!
This goes back to her early experiences she was the only survivor of a litter ( not yet weaned) and she was ' nursed" by a Tech at my Vets office - I suspect because she started life in a kennel cage,alone and probably was only handled to be fed,poked,prodded n medicated that's why.... So yeah,it's very important they get lots of handling with love, gentleness and not hurried while they are little