Woof

If you don't fix a dog, you run the risk of it running off when a bitch goes on heat, and quite possibly getting mauled by another dog. That's how I lost my old dog Bo.

Not to mention they tend to get more aggressive if they're not neutered, especially toward other male dogs.

they are also less aggressive when fixed

Then I hope you never get a dog.

why would humans do such a thing?

FJ, the place where you get thumbed down for asking a question.

Apparently there is a high risk of cancer and other complications if they have unused balls

i guess so they would stop humping stuff :P

And they cost a lot more to keep them on if they're pure bread.

Pure bread dog

Actually, there's a number of reasons, xgeneration
One: the uncontrolled breeding of stray dogs has left the world with infinitely more pets than there are homes. With a male dog that isn't fixed, you run the risk of him impregnating a female stray, who doesn't stick around for you to take care of the puppies. These puppies grow up and continue the vicious cycle, leaving many poor animals alone in the wild where their physiology is no longer really designed for, causing the number of feral animals in suburban areas to skyrocket.
Two: Male dogs who aren't fixed tend to be extremely aggressive. They become territorial, and much more likely to attack other dogs and even humans. While not all male dogs are like this, the risk is too high to safely go without fixing the dog.
Three: It also prevents testicular cancer in male dogs, and fixed female dogs also tend to live longer, healthier lives.
Four: It makes your pet much less likely to run away in search of a mate.
There are plenty of reasons, and fixing your dog won't make them stop humping. That's a dominance thing, and there will always be something your dog thinks it's dominant over.

take care of your don or he'll take care of you!

Can you be there 24/7 for that animal? Can you make sure that he doesn't get a spur-of-the-moment desire to fuck up someone that came too close to the yard? Can you be absolutely 100% sure that your dog won't get out of your yard or your control for the few minutes it takes for him to track down a bitch and impregnate her? And what of the actual health benefits? Preventing testicular cancer seems like a good fucking start to taking care of your animal.
And what happens, anonymous, when your dog does get away? If by some chance, he makes a break for it? If he's not fixed, it is literally impossible to ensure that he won't try running off for the fuck of it. Even if he is fixed, it's still a possibility. At least if he's fixed he's more likely to return to the comfort of your home, and won't be able to knock-up some stray female who will in turn continue the seemingly endless cycle of stray dogs living their entire life in an environment that is brutally dangerous for them.
But hey, let your dog keep his balls. I'm sure your little humanitarian/machismo intentions will make it all okay when he puts another litter of feral dogs on the streets to starve and suffer.

You really don't get it.
Your pet dog is a pet. Remember that. Because of that, it's down to you to decide the better course of action.
And it's not akin to a woman getting her breasts removed because she attracts men, it's like her getting her tubes tied because she wouldn't be a fit mother, and would only add to the already booming population of orphans. It's like a man getting a vasectomy because he plans to remain sexually active, but also knows that he won't commit to a woman and wouldn't burden her with children he won't be there to take care of.
And you continue to ignore the aggression factor here. While yes, some dogs can retain their testicles and not become overly-aggressive and territorial, and while the inverse is true where fixed animals can still be aggressive, it's a matter of not taking a stupid risk over something so arbitrary as your dog's testicles. You would run the risk of your dog being put down by the state because he mauled some random chick's chihuahua, because you're convinced there's some actual reason for letting your dog keep his testicles, when there really isn't. There is no lost pride, there is no decreased drive, and the correlation between neutering your dog and increased weight gain is non-existent and completely depends on your ability to keep your dog exercising regularly.

How the hell a reaction picture can be so relevant?

Niggerkaiser approves

this pic of yours made me think of how i always tell people i wanna get 2 dogs and name 1 "cock" and the other "sucker" just to see the look on peoples faces while i stand on my porch yelling "cock sucker, come here cock sucker"

and changes breed

And that..didn't think of that..

is that kinda like the michael jackson disease but for dogs lol