Well as long as the emo floodgates are open...
The thing about dogs that trumps all else is that dogs will give you their unrequited love and loyalty. Sure they are clumsy goofs and they can smell doggy at times, but for dog people this is a very small price to pay for the palpable adoration that you receive from your pet dog. This all sounds very selfish and one-sided, but that brings up the other side of the issue: dogs love loving you. You absolutely make their day just by hanging out with them. If you have the time to invest in making a real best-friend-for-life, it's definitely worth it. There's a dog breed for everybody.
Very low-maintenance, the closest you can come to this kind of a relationship otherwise is with children/parents. Obviously I don't endorse it at all, but the fact is that any situation where you can do violence to something and it still continues to loves you is powerful magic. Similarly, even a terrifying level of neglect is usually not enough to easily make dogs lose their loyalty. As lame as Animal Planet is, I think it does a good job of highlighting the insane conditions dogs are willing to put up with for the privilege of protecting their master. Immediate relatives and dogs are about the only things that behave like this.
Below that, there's a touch of the same behavior in Cats/Spouses/Lovers. I mean Rihanna went back to Chris Brown, right? At the start of a relationship perhaps, or if you have little self-esteem, or if you're dealing with a cat, then you'll see the same fragile degree of love. This is pretty violable though. You push someone too much and they won't forgive you. There's nothing necessarily wrong with this either. I wouldn't really want a lover who was totally into me and couldn't be pushed over the edge by me being an asshole over and over again. The thing is this kind of love can fade without you even doing anything wrong. There's no guarantee of permanence here. Not like with dogs.
Dogs are loyal to their masters to the death.
PS - OK maybe I'm selling cats a little short. They can be neglected or hurt and they still keep coming around. The thing is I'm not so sure it's not just about food for most of them. They're selfish little bastards. If you offered companionship but not food, the cat would blow you off.