In Pirkei Avot 2:9 it's Rabbi Elazar Ben Arach who states that a Lev Tov (good heart) is the greatest good - the best "acquisition" - or best trait for a person to acquire, or which should be the goal or aim of a person in this life. More literally, "unto which good way should a person cleave?" it's a Lev Tov. (Based on the question of Rabban Yochanan Ben Zakai: איזוהי דרך ישרה שידבק בה האדם).
So the straight path,or walking a/the right path, consists of having a Lev Tov.
My question is: How does one develop a Lev Tov?