Rabbi Pinchas of Koretz (one of the great disciples of the Ba'al Shem Tov) said that a consummate tzadik (righteous person) is one who loves a consummate rasha (wicked person) and that a not-yet consummate tzadik is one who loves a not-yet consummate rasha.
The tzadik sees himself reflected on the other side. He identifies with and loves that reflection of self.
In physics, every particle has its anti-particle – particles exist in pairs (e.g., an electron and a positron). When they meet they cancel one another out, their union takes them back to their common origin – nothing.
What is the difference between a tzadik and a rasha? The tzadik recognizes the rasha as his spiritual counterpart (on the other side) and desires to unite with him in love, to return with him to the Divine nothing from whence their souls issued. But not so the rasha, at least in his initial state of consciousness. He feels infinitely estranged from the tzadik.
By manifesting more and more of his love for the rasha, the tzadik nurtures the seed/potential buried deep in the soul of rasha to recognize his holy counterpart and long to unite with him.
That's why some of the greatest tzadikim and greatest reshaim were best friends.
As they touched one another in friendship their independent realities ceased to be – no longer an aloof tzadik and no longer a disengaged and estranged rasha. Just two brothers, born from a common womb.