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" is a Lev Tov. (Based on the question of Rabban Yochanan Ben Zakai: איזוהי דרך ישרה שידבק בה האדם)
So the straight path, walking a right path, consists of having a Lev Tov.
In the same way Rabbi Elazar said that the evil path, the wrong way, which one should distance from, is a bad heart (Lev Ra).
It almost seems as if Rabban Yochanan Ben Zakai was asking: What is the gate via which one can reach an understanding of the mitzvot and their fulfillment? What path will lead to a true life of Torah, faith and service?
Did Rabbi Elazar answer in the sense that one must choose good (to do mitzvot aseh; act upon that which is commanded; keeping Torah, walking in His way) and stay away from bad/evil (not to do mitzvot lo ta'aseh; taking care and ensure that one doesn't commit a sin, refraining from that which is ra, etc.)?
What do these terms Lev Tov and Lev Ra imply?