"A person should always excite the good nature against his evil inclination" (Berachot 5a).
This does not mean that a person must at every moment break his heart and experience a bitterness of soul.
That is not the way to serve God or attain holiness.
Rather, a person must do so only at certain times.
A principled person may grow proud and pleased with himself.
At times, this may inspire him to progress with a sense of holy joy.
At other times, however, this feeling brings on smugness that prevents him from seeing his flaws.
Then he does nothing to improve.
Such a person is rotting within, sinking into the torpor of the dull heart.
He should excite his good nature against his evil inclination by exploring whatever is broken and damaged–by examining himself.
As a result, he may awaken with a renewed vitality.
These are worries that cause one to be sad, such as monetary problems, mourning, anxiety, false accusations, and so forth.
People go through cycles of emotions and thus may grow sad without reason.
But whatever the cause, when a person is sad, that is a good opportunity to experience sadness and bitterness in the realm of holiness.
When a person is sad for whatever reason, that is the right time to do soul-searching.
Introspection may lead to a bitterness of the soul and dejection about his spiritual concerns.
This can change his mood from a sadness pulling him downward to a bitterness of the soul that can lift him up.
One can reflect in a number of ways.
But however one proceeds, one must make sure that it will be elevating and energizing and not the opposite.
Thus, in the realm of this-worldly matters, a person should compare himself to those who are less well-off and to learn how to be satisfied with less.
In the realm of the spirit, by contrast, he should compare himself to his betters, to realize how imperfect he is and to feel the urge to become more holy.
Unfortunately, many people do the opposite.
In regard to this-worldly matters, they compare themselves to those who are better off than they are and as a result grow depressed and envious.
And in the realm of the spirit, they compare themselves to their inferiors, as a result of which they become self-satisfied and dull-hearted.
—Rabbi Adin Steinsaltz
From Understanding the Tanya, Chapter 31, by Rabbi Adin Steinsaltz