WHY do I have to get my 4 month old chihuahua neutered?

If he should ever get loose, you can't stop him from mating. If you can maje 100% certain that will never ever happen, then keep him intact. UNLESS... you signed a spay/neuter contract. To not follow through will result in reposession of the dog.

You don't HAVE to. Neutering your dog would help its behavior and temperment and would make it a nicer dog overall. Also, when a dog isn't neutered, sometimes they try to stray away to find a mate. Always inside? Dogs need walks?

You don't HAVE to, especially if you take care that the dog doesn't escape and has contact with females.

But it eliminates some possible behavioral and health issues. It also eliminates the chance of the dog reproducing. It's nice to have for peace of mind.

My vet told me that if you spay and neuter animals early, before sexual maturity (6 months old), it prevents prostate and breast cancers, in addition to testicular cancer and uterine cancer (which spaying and neutering at any age prevents). Unfortunately, I can't find any scientific studies to back that up.

Neutering also helps prevent marking/spraying, which male dogs sometimes do when they reach sexual maturity.

You don't.

But you don't ever allow that dog to run off and impregnate a female... and if it is a female then it is a wiser choice to have her spayed as there are too many nasties that can occur to an unspayed b*tch.