Zöe licked his eyeballs every single day. She also licked the corners of his mouth and his nostrils. Zöe was Bronco's mother and a very good mother at that.
Among the mammals, many non-human mothers clean their babies that way, but this was the first time I had observed a dog continuining the practice so consistently after the infant became an adult.
I don't know if it is an erotic thing and if it is practiced mainly by the female of the species. For myself, nostrils have always been a vaguely erotic part of the human body. I don't know if this is my femininity manifesting itself or a primordial feeling of maternity.
Zöe was my wire-hair fox terrier; she died a few months ago, shortly after Bronco's 10th birthday. Since then I have cleaned his eyes every morning; he has a chronic infection in one eye. Bronco doesn't seem to particularly enjoy it but, being an intelligent dog, he appears to know it is a necessary part of his grooming. (I use an eyewash, not my tongue.)
On occasion he will try to lick the corners of my mouth. This, however, is greed on his part as it usually happens after I have eaten a biscuit or some such tidbit. The scent of the food on my lips is what apparently attracts him. Men!