One only takes a pledge from someone if he believes they owe him. Never would one take a pledge from someone that he owes. How much more so when it comes to the Creator of the Universe who at every moment of every day supports us, sustains us, provides for us and protects us. As Rabbenu Bachya writes " If all the good works of all the people throughout history were to be credited to one individual, all these would not suffice to repay the Creator for even one of His favors towards him." How ridiculous then would it be for someone who already owes more then he has to the Creator to then ask for a pledge and put himself in more debt. In his lifetime he would never be able to repay all the good that H'shem has done for him.
pg 459
pg 459
