Question

Why doesn’t Hamilton want to interpret the constitution loosely

Answer 1

Because the constitution doesn't exactly say that he can create a national bank to complete his financial plan for the United States. 
Loose interpretation is when you believe that the thing you are trying to do is necessary and proper than congress can carry it out. 
Strict interpretation is when you go directly off of what the constitution says. 
If he chose a loose interpretation then he would be able to establish the bank