There is, consequently, an enormous range of daily life that does not fall explicitly under the obligation of a “duty of the limb.” However, the Chovos Halevavos explains (and bases his sefer on the concept) that there is an entire field of opportunity called the “duties of the heart,” which encompass a person’s inner world, his thoughts and his attitude toward everyone, everything, and every situation. A person, for instance, can say a blessing over food, but does he feel gratitude in his heart that his Creator made such food? That is a duty of the heart. A person can fulfill by rote all the daily obligations of the Shulchan Aruch, but is he happy simply to be alive and have the privilege to serve his Maker? When one becomes conscious of the “duties of the heart,” the Torah is truly endless.
Astor, Yaakov. Rav Avigdor Miller on Olam Haba (p. 340). The Judaica Press, Inc.. Kindle Edition.