Ok. Why not to breed your dog:
Where did you get your dog? Did your breeder show her dogs in conformation shows? If your dog isn't from top notch stock you're not going to be producing quality puppies.
Are you showing or preparing to show your dog? The purpose of showing is to select the next generation of breeding stock.
How much money do you have? You're gonna need a lot. Before you can responsibly breed your dog she must be 2.5 yrs old (minimum), scanned for MVD by a certified cardiologist (and will need this annually), MRI scanned for SM (a few hundred dollars if you can take advantage of a low-cost scheme for breeders, $1500-2000 if not) MRIs should be redone a few times during her life, eyes should be checked by an ophthalmologist, hips need to be x-rayed for hip dysplasia, patellas need to be checked (your regular vet can do hips and patellas)
Then, hopefully everything will go well, I've seen several puppies get stuck in birth canals, if you are able to recognise the problem and get her into a vet for a c-section she'll have a good chance of surviving, usually the puppy who got stuck is dead by the time the vet can do anything, and oftentimes 1-2 other pups will die from the ordeal. It's also really expensive, especially if this happens at night and you are paying emergency vet fees.
Unspayed females risk developing pyometra (an infection of the uterus), basically they'll go into heat, bacteria or fungus gets into the uterus, she goes out of heat and the infection gets sealed in, 8 weeks later you're racing to the vet (or emergency vet at night) for an emergency spay (bring your wallet). Pyometra is always painful, and often the infection has progressed too far and the only option is to put the dog down.
I'm sure others can add to this post, but those are a few reasons.