Fleas and ticks are parasites that live and feed off of the blood of a host, which in this case is your dog. They need to ingest blood in order to survive and reproduce.
Dog fleas have evolved over thousands of years to have specialized appendages and biology in order to successfully feed on the blood of dogs.
Immature fleas in the pupa stage are wrapped in a cocoon and can lay dormant for months or years waiting for the presence of a dog to appear, so that they can come out of dormancy to feed. They can lie hiding in the grass or carpeting or other discrete locations undetected.
Dog fleas have the ability to sense the presence of carbon dioxide in the air, as well as sense tiny vibrations in the air and ground that may signal the presence of a suitable host.
When your dog comes in contact with another animal or an area that is infested with fleas, the fleas are transferred to your dog and they begin to reproduce and lay eggs on your dogs skin and fur.
This is how your dog gets a flea or tick infestation.