I would really like a dog but my parents won't let me... How should I persuade them?

I am 13 years old and I am an animal lover. I love researching about them and I just adore animals. I have always wanted a dog but I haven't brought it up till know because I knew before that I was too young and irresponsible back then. Now, I know that I am ready and that I will be able to take good care of it. I have saved up all my allowance till now for this moment and guess what?... They said no. I was planning on going to boarding school in High school but my parents wanted me to stay because they said that after my brother goes (next year) to America, they will be lonely. This means I still have a lot of time until I have to leave to college because I am in 6th grade right now. I've been asking them respectfully in a non-pestering way but I am getting a little impatient. My uncle used to keep a schnauzer named Locco. I used to feed him, play with him and wash him but now, Locco is at a farm. Please give me some ideas to help me. If you agree with my parents, please give me your reasons so I can think over too. Thank you for your help!