Sometimes the most truthful and down to earth response can be the best reason. And that probably is that you really want one and it would really make you happy to have it.
Truth be known, there is not any solid winning argument, other than just plain old simple companionship, for anyone to have any sort of pet.
Obviously you are on the young side, so you just need to be certain that you have enough responsibility to FULLY care for your new pet. That might really be your dad's main concern.