Hi,
Some dogs lip line curves in a natural smiley position but dogs can actually smile too. A proper grin is a better discription of most doggy smiles. Interestingly wolves in the wild do not smile but captive wolves learn the expression- where from? The only reasonable answer is from the humans that care for them. Dogs and wolves never use the same expression to each other, the same mouth shape occurs but not with the same intention or meaning. So yes dogs can smile.