While it's true that pets require a deal of responsibility, time, and effort to keep, pet owners are amply rewarded. Studies have shown time and time again that caring for a pet actually reduces stress in people's lives -- staring at a fish tank, wrestling with a dog, or cuddling with a bird are all beneficial to mental health. Indeed, some hospitals actually have on-site dogs for post-operative patients to play with; the company of any friend, a person or an animal, makes people happy.

Dogs, Cats, and Small Mammals
If you live in a nice house with a big yard, have lots of time and money, and need a pet that will provide you with the[arguably] greatest love, look to dogs. Dogs are great pets ("man's best friend" over here). On the other hand, a lot of people don't have ALL these things. So how about a cat?
I hate cats. If you don't really care to receive affection from your pet, need a dumb, untrainable animal, enjoy being treated as property by what should be your pet, and are willing to pay the food, housing, toy, and vet bills for such a beast, maybe you want a cat. I suppose I'm quite biased though.
Gerbils, hamsters, guinea pigs, and rabbits are cute little furballs, and they come with slight space, money, and time requirements. Unfortunately, they aren't known for their playfulness, and each of these pets has its own drawbacks; for example, guinea pigs can bite hard (and as your finger looks like a tasty carrot, they might do it way too often), and hamsters live for only two or three years (so don't get too attached).

Birds
And now birds. What if you:

don't	want
have much space for a pet	a pet that shows affection
have much money	to play with your pet
have lots of time	a cute pet
want to deal with the loss of a pet often	a pet you can train
mind new responsibilities	a beautiful pet
	to own a dinosaur

...then a pet bird -- specifically a pet 'tiel -- would be a perfect companion for you.

A tame cockatiel can be very affectionate. It loves to be petted and cuddled. Every morning when I open her cage, my cockatiel, Tuolumne, stretches her head out to my finger and nuzzles up to it so I'll scratch her head. After awhile she'll twist her face around and start rubbing my finger with the side of her beak. Yep, my pet bird pets me.
Cockatiels are very social by nature; they survive in the wild by hanging around other 'tiels in a flock. So, they get along well with everyone, even to the point of devotion. It's pretty cool when your bird is on your shoulder and starts to clean your hair with it's beak (preening) and snuggles up to your ear. Tuolumne's wings are clipped, but she positively leaps onto my shoulder when she can (she isn't so enthusiastic hitching rides on other people, though); I had no idea 'tiels can jump so far.
Unlike virtually all pets but dogs and other birds, 'tiels can be taught to "come" on command. I know a 'tiel named Angel who sort of flew (her wings were clipped) to her owner whenever he called. Heck, lots of times she hopped or waddled to him without being called. Tuolumne won't come to me on command from far away right now (maybe I'm not just worth the effort from more than a few yards away right now). I'll note, though, that not all 'tiels are perfect pets; much of the issue involves an individual's personality, and it's best to get one young and hand-fed.

Maybe birdbrain should be a compliment. One really wouldn't train Alex, Dr. Irene Pepperberg's African Gray project. Alex learns. He can do math. "Alex, what's two plus three?" He can identify objects based on characteristics Dr. Pepperberg suggests, like color or shape; "Alex, pick the blue square." He has opinions (he says, "Want walnut," and tosses the other kind of nut someone had given him). For more information go to Alex's web page or NOVA, Newsweek, or Time.
Of course, a 'tiel is only a distant cousin to an African Gray and isn't nearly as intelligent, but cockatiels can certainly be trained -- taught. They can come, shake hanks, bow, and do other stuff on command. And naturally, they can be potty-trained. Really, cockatiels only come second to dogs or parrots as far as trainability in the pet world.

To be honest, if I could choose any animal from the pet store for a pet, it certainly wouldn't be 'tiel. It would be an parrot or cockatoo, a bird in the hundreds or thousands of dollars. But this is the real world and I haven't even gone to college yet. But let's ignore the issue of money first.
Most pet birds fall into one of two categories, the Psittaciformes (parrots and parrot-like birds) or the Passeriformes (canaries, finches, and such). The passerines are neither remarkably social or intelligent, and do not make really good pet birds (unless you're concerned only with singing ability or looks). Psittacines, on the other hand, possess both those qualities.
So a parrot or parrot-like bird would make a good pet. But which kind? Now price comes into play. budgerigars (parakeets), lovebirds, and cockatiels fall into the low end, and the others form the high end ($200+). For most of us, that means we'll be buying a budgie, lovebird, or 'tiel. So here comes the final stage of elimination.
Budgies are very inexpensive, less than $20, and they talk. Unfortunately, they aren't as intelligent as lovebirds and 'tiels, and are not as affectionate as 'tiels. They do come in copious beautiful color mutations, however. Lovebirds and 'tiels are generally $50 to $100 (more for very rare colorations). Lovebirds are more colorful. 'Tiels are more social and can whistle adeptly; also, they are much more gentle -- even when they do bite, it's much less painful than a budgie or lovebird bite. So it's a looks versus love issue. Since cockatiels are cute anyway (and they have that cool crest!), they're a great choice.