For certain things (personal things) I agree.
You wanna use a bit of your paycheck that isn't needed for the household to get your favorite cupcakes, get a haircut, new lipstick..
TREAT YOSELF'! as they say.
but a dog affects everyone. Like it or not, the dog will be interacting with, be around, likely be cared for or at least fed/watered/walked occasionally, by other people in the home.
Not to mention..screaming puppy during house training.
I would sit down with your s/o and really tell him how you feel. People who aren't dog people sometimes don't "get it". Like.. ok, she wants a dog like I want a sports car.
They don't understand that dogs are for some people a big part of our daily lives and who we are. Having a dog as part of a daily routine is more than just "nice", explain to him that it brings you comfort, joy, happiness.. etc..
Perhaps discuss your plans to make the new puppy as non intrusive/problematic as possible (budget, doggy daycare, things you already have, vet plans, etc..)
Good luck