Why is There a Precise Number of 613 Mitzvos?

The Torah is like the Kesubah, marriage contract, between Hashem and the Bnei Yisrael. Just as a contract has to be very specific on the conditions of the agreement so that there can be no misunderstanding, so also, the Torah has to be specific about the obligations of Bnei Yisrael. Therefore, we are told that there are 613 commandments for which we are responsible.