For me is was an age thing....and the fact we had to go through IVF to get No2. If not for that I would have loved to have at least one more.
For DH it was money. Even when I said I want to go for No2 he was hesitant. He never had much growing up either so I think he wants to make sure he can enjoy life as he gets older (he also has two other kids he supports).
It took a while for him to understand my need for another bubs and the fact that I don't think money is an issue (we are both working and on good incomes) and once he finishes paying for his older kids in a couple of years we will be in a very comfortable financial position.