Obligatory blessing means that the body obligates him to bless. That is, the body that the Creator created with a nature that if he receives good things from another, he blesses him. This is called “The blessing for the food from the Torah.” It is not a Mitzva [commandment], meaning that the body does not obligate him to bless, since he feels that he still needs a few things that the other could give him but is not. At that time, the body says that it is still not satisfied with him and cannot say that He is good and does good, since he needs a few more things, and why is He not giving them to him? Nonetheless, he is strict with himself and blesses. It follows that he is not commanded yet he does.