Tail Wagging

There isn't a more certain sign of happiness than a wagging tail, but fortunately this sign is reserved for dogs. How much of a pain in the ass (literally and figuratively) would it be if every time we you were happy, a limb-like appendage at the end of your butt whipped around full speed. Not only would it make walking through narrow doors painful, but it would also make hiding your happiness impossible.
For example, say one of your friends beat you in Mario Kart last weekend, and gloated about it for days. But today he came over to tell you that he lost his video-game thumb in a terrible grapefruit spoon accident. The socially acceptable response is to look aggrieved and say something like, "Holy crap, I always knew those were a dangerous breakfast food." But inside you're overjoyed that he'll never be able to beat you in Mario Kart ever again. And if you had a tail, he'd know just how happy you were to hear about the death of his thumb. You don't want him knowing how cruel you are. Or how seriously you take Mario Kart.
(flickr photo Suppertime by basykes; http://www.flickr.com/photos/basykes/967289880/)

Sleeping All Day

Dogs and cats sleep all goddamn day. Seriously, all day. Some stats I read from semi-credible sources said that cats can sleep up to 20 hours a day, and dogs can sleep 18. And although they don't always sleep that long, the average for both is well above 12 hours. 12 hours! These lazy, good-for-nothing, over-rested fuzzballs. Hippies don't even sleep that much. It's appalling...and I'm jealous.
(flickr photo Why I love my new Canon PowerShot G9, Part 4 by sskennel; http://www.flickr.com/photos/sskennel/2368230257/)

Paw Licking

Obviously, you don't have paws, but you would never lick your hands. You'd either look moderately insane or incredibly perverted. Mostly perverted.

But the perversion isn't even half of it. When cats lick their paws, they do it after touching all sorts of disgusting things, and they don't use antibacterial soap beforehand. They don't even use probacterial soap. That's just downright gross. If I touched a toilet and then licked my hand, most people would vomit a little in their mouth, but cats can do it all day long and no one thinks twice.

(flickr photo "Clean Cat" by Rene Ehrhardt; http://www.flickr.com/photos/rene_ehrhardt/2391415460/)

Butt Sniffing

Say you're walking down the street, thinking about that delicious burrito you're going to eat after you wash your favorite pair of Christmas socks, when you see one of your closest friends walking right at you. You reach up to give him a Top Gun style hi five, but he evades it, spins around you, and plants his nose right in your ass, inhaling deep like a smoker on their lunch break. Now pretend you're not wearing any clothes when he does this. And that you like it and do the same. And that afterwards you both lick your noses.

Well dogs do this all the time. Everyday, multiple times a day. They love it. Dogs can't wait to sniff another butt. If your dog had money, he'd probably pay you to let him sniff yours. Kinky, I know, but really it's just their way of saying hi. But if you did that you'd get punched, slapped, yelled at, or tossed in a padded room.

(flicker photo Arse sniffers by Ross Angus; http://www.flickr.com/photos/ross_angus/767911035/)