1st look at your assets. Do you do your chores or have any? Do you help around the house? If so you have an advantage. Doing chores shows responsibility. Tell them you want a dog to help you be more responsible as an adult and future caretaker of others including your future children. If they give you that dog, then HOLD UP YOUR END by taking care of your pet. ENJOY.
A dog is a lot of responsibility. I would suggest bringing up the idea to them then showing them that you can be responsible and take care of things without being told.
You don't as you prove to your Parents that you are responsible to maybe entertain the idea of getting one. A pet is a lot of responsibility and that responsibility will lie with you to feed and water it, play and exercise it, clean up after it, bathe it and comb it, and not when you feel like it, but everyday, rain, shine, snow, whatever, holidays, birthdays, partys, movies, etc. A pet depends on you to take care of him since they can't so show your parents that you're responsible enough to do this and just maybe one day.
Since you do have other pets, maybe it is due to lack of space or money. If they are worried about space, there is nothing that you can do. If it is money they are worried about, work really hard to raise enough money to cover the fees of adopting one, vet fees and several months worth of food. You need to show them that you are capable of knowing how much a dog will cost and that you are willing to do whatever it takes to make sure that they don't have to shoulder the burden of paying for a dog.
