Fleas like to torment and bite your pets so much because they are parasites, and as such feed on the blood of hosts, in this case your pet.
Fleas need blood in order to survive and reproduce, and in order to obtain this blood they bite your cat or dog.
If a juvenile flea emerging from the pupa stage does not successfully obtain a blood meal within about one week, it will die, so it is imperative that it finds a suitable host in a short period of time.