In this week's parsha we read, "Do not hate your brother in your heart; rebuke your friend and do not bear a sin 'upon him'" (Vayikra 19, 17). Rav Wolbe (Shiurei Chumash), based on the Ramban, offers two explanations of this pasuk, rooted in the two causes for hatred between man and his fellow man.
The first possibility of hatred is caused when one observes another person transgressing a sin. Although this would seem to be a valid reason for hatred, nevertheless, the Torah tells us that the observer may not hate the transgressor; rather, he must rebuke him for his misdeed. Moreover, if he fails to rebuke him, the Torah considers it as if the observer himself committed the transgression, and he will be punished accordingly. In this vein, the pasuk is to be explained, "Even if you see your friend sin, you should not hate him. Rather, you should rebuke him, for if you don't, you will bear the sin!"
The second possibility for hatred is caused when one feels hurt by another's actions. It doesn't make a difference if he was hurt physically, emotionally or monetarily; the Torah tells him that he should not bottle up the resentment inside himself. Rather, he must discuss with the offender the motive for those actions, thereby giving him the opportunity to explain or excuse himself. However, if he hides the hatred inside his heart, he is placing a sin upon the offender because he may not know the gravity of his actions, or miss out on an opportunity to ask for forgiveness. According to this explanation, the pasuk means, "If you were hurt by another, do not keep your hatred inside your heart. Rather, speak to him and discuss the reason behind his actions so that the transgressor does not end up with a sin."
Rav Wolbe adds that often it is much easier to just go on hating someone, even for years on end, than to speak to him directly and ask him why he acted in the way he did. It is very hard for the one who feels slighted to initiate a conversation that will give the offender the ability to excuse himself, thereby obviating any reason for hatred. Yet the Torah commands us not to let the hatred fester inside our hearts. Rather, we should speak up and discuss it so that we can ultimately fulfill the obligation stated in the following pasuk, "You shall love your fellow like yourself."