Brushing pet's teeth: What do you think?

Depends on your dog and your vet...a dog that chews and has a human to help take care of his teeth may not need to be cleaned as frequently as a dog who doesn;t chew or who only eats wet food. Your dogs diet does have an impact of his teeth...my little girl needs her's done for the first time and she just turned 5

Surely just giving them bones will do that? Im sure they didnt have toothbrushes in the wild, and you may make them more sensitive.

Brushing your pets teeth is a good idea if you can get them to. Otherwise bones and chews and regular vet teeth cleanings would be good. And a pets teeth are like people's, if you don't clean them they will have periodontal disease too which can have an effect on the body as a whole. If you see bleeding in their gums, just like in human gums- it means there is an active infection present and the bacteria needs to be removed to have complete healing. It does not mean don't touch the area because it may bleed just like when people don't floss because "it makes their gums bleed". It's bleeding because of the bacteria they have allowed to build up causing the infection- same as in dogs and cats and other animals. I know this because I'm a dental hygienist.