Another baby?

Okay, I have a two year old son, and a one month old baby girl. My husband is wanting to have a vasectomy, doesn't want more children. And, that's what I thought, too. But, the more I think about it, the more I want just one more. Not right now, but later on. But I can't help but think it's because mine will no longer be an infant. How do you know if you want another because you want another, or because you miss having a baby around? Am I making the right decision?