Of course they wonder why we don't smell their bums. It's obviously the height of canine rudness not to.

Actually the REAL reason is spelled out in "The Dogs' Meeting" where a bunch of dogs go to a concert and have to hang their (knock, knock, knock)s on a hook. A yeller dog hollers "fire" and they all grab the nearest (knock, knock, knock).

....And that's the reason why, my friends,when walking down the street
A dog will stop and swap a smell with every dog he meets
And that's the reason why a dog will leave a big fat bone
To go and smell a (knock, knock, knock) in hopes to find his own.
Cheers, Joy