Every living thing in the world has potential, and it is the Divine will that everything achieve its maximum potential. We think of humans as the only beings that have a yetzer hara which causes them to resist growth. Certainly animals and plants, which do not have a yetzer hara, should achieve their maximum potential quite easily.
Not so, says the Midrash. Even plants, and in fact all living matter, have an inherent "laziness," a tendency towards inertia. Even the lowly blade of grass needs to be stimulated and urged to grow.
We can see from here that a human being thus has two inhibiting forces to overcome in order to achieve growth: (1) the yetzer hara, which is unique to us, and (2) the force of inertia, which is common to all matter.
The Tanya postulates the existence of absolutely righteous people who have totally eliminated the yetzer hara from within themselves. We may ask, in the absence of even a vestige of yetzer hara, how can they grow? The answer may be that they strive to overcome the inertia that is inherent in all matter, including themselves.
If a lowly blade of grass has both a tendency towards inertia and a spiritual "mentor" which demands that it fulfill itself, we human beings, with two adversaries, certainly have even more powerful forces urging us to achieve our full potential. We should be aware of what can hamper our achievement and make the effort to overcome it.