Dogs are humans best and most loyal friends. Sadly, the humans often betray them for commodity, lack of time, a new human baby, moving out or irresponsibility and also ignorance and evilness.
Millions of millions of dogs end up abandoned on the streets, parks, forests and even deserts, to have a slow and cruel death. Others are taking to a kill shelter, dumping the dogs as unwanted garbage. The dogs do not understand what they did to be left and abandoned, so they feel anxious and depressed.
Thousands of pedigree dogs are bred for profit and sold for huge prices. These places known as "puppy mills" exists for the whole purpose to breed and exploit the mothers for several years, until when she cannot produce more "quality" puppies, they are abandoned or killed. Literally they breed them to their death. Runts of the litter are also killed.
For these reasons, it is always important to only ADOPT from licensed shelters and good reputation charities. Never BUY any dog or any other animal. Whenever you buy a pet, you contribute to the ongoing exploitation of these animals.
Love them as much as they love you! They will be your best friend forever!