Rebbe Elimelech of Lizhensk zt'l chose one middah to work on, and he toiled for at least twenty-five years to acquire perfection in that particular middah. His students asked him why he focused so much on that middah. He replied, (a) Perfection in one middah, alone, greatly purifies a person. (b) When one attains excellence in one middah, he automatically attains perfection in all other good middos.
All good middos are connected. Therefore, tzaddikim would focus on perfecting a single character trait, and automatically, they attained excellence in all other good middos. A hint to this concept is the Chazal (Midrash, Mishlei 1) "If one keeps one mitzvah perfectly, it is like he kept all 613 mitzvos." This is because all mitzvos and all good middos are connected. Perfection in one area generates perfection in all aspects of avodas Hashem.
For seventeen years, the Chidushei HaRim zt'l toiled with all his strength to attain an טובה עין, a good eye. He succeeded. He said that no one ever acquired this trait as perfectly as he did.
One aspect of טובה עין is to see only the good in others. When the Chidushei HaRim zt'l became a chassidic Rebbe, he had to see the faults in people (to help them improve). This bothered him immensely.
The Chidushei HaRim writes, "During [the Three Weeks], one must eradicate sinas chinam and all aspects of a bad eye. Even if he doesn't look down at anyone, if he doesn't focus on their good, that, too, can be called sinas chinam... With a good eye, the Beis HaMikdash will be rebuilt."
