When someone asks you, (in general) when it comes to this question of “Why should a Jew live in accordance with the Torah?” How should one respond to that person who asked you? What’s the best way to respond back to them?
The answer to this question depends greatly on the identity of the person asking the question, and also on the identity of the person being asked the question.
The basic answer is as Jews, we should live according to Torah law, because this is what being Jewish is all about. We are Jewish, writes Rav Saadya Gaon, because of the Torah we observe — not because of Yiddish songs, not because of chicken soup, and not because of universal humanitarian values.
Beyond this, however, the Torah promises us good both in this world and, primarily, in the next world, and if we wish to achieve self-perfection as Jews here, and eternal reward there, we need to follow the Divine instructions of the Torah, and to merit the closeness to Hashem latent therein.
The Chovas Ha-Levavos bases his answer to the question on the concept of gratitude: We are indebted to Hashem for all we have, and this is an adequate basis for compliance with His directives. The Rosh refers to the “oath” that we swore at Sinai — an answer that is less likely to be persuasive in a modern context.
It is hard to elaborate on this matter, becasue as noted, the answer will change depending on identities of the questioner and the person asked the question.