I'm going to assume that no one needs to be told what human belly buttons look like. But most people probably don't know what dog belly buttons look like. Here's a photo of one, which should make it easier to understand why we usually don't notice them:
I'm guessing that the difference between the two has something to do with the way the knot is tied, but is there a more thorough explanation? Why are dog belly buttons so different from our own?