As we go through life we have many challenges which Hashem is specifically tailoring to every single individual in order for that person to become a better human being and bring out G-d’s greatness into the world. A person grows through overcoming these hurdles and realizing what the real world is about.
When someone insults me, instead of focusing on the negative aspect of the incident, let us focus instead on how does Hashem want me to deal with this? and then accordingly sometimes we have to retaliate and sometimes keep quite. Either way, if we understand that it comes from Hashem, even though there is room for negative feelings, however it won’t turn into hatred.
This is the “needless hatred” the Rabbis are referring to. Even though it seems there might be grounds for it, to hate due to Hashem’s challenges is considered “needless”.
This is not an easy task but proving ourselves in this manner will bring the rebuilding of the Beit Hamikdash במהרה בימינו אמן.