All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Generally, a person has a natural disposition in socialising with people. In doing so, he is subjected to fall into what causes disagreements and disputes.
However, Islam has guided us to some principles of good morals and standards, among which is the saying of Allaah (which means): {And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and there upon, the one whom between you and him is enmity [will become] as though he was a devoted friend.} [Quran 41:34]
Moreover, the Prophet said: "Deal with people with good moral standards." [At-Tirmithi and others]
Therefore, if a Muslim has good moral standards, he gets the highest degrees in Paradise. The Prophet said: "I guarantee a house at the highest part of Paradise to whoever has a good moral character." [Abu Daawood]
Hence, if a Muslim has wronged another Muslim brother, like insulting him or scorning him, then he has to seek his pardon. Otherwise, the right of the person who was wronged will be taken by Allaah from the one who wronged him on the Day of Judgment. The Prophet said: "Whoever wronged a Muslim brother has to seek his forgiveness." [Al-Bukhari]
Similarly, the person who is wronged should forgive in order to obtain the Forgiveness of Allaah. Allaah says (what means): {… Let them pardon and forgive. Do you not love that Allaah should forgive you?…}[Quran 24:22]
The Prophet also said: "Indeed Allaah does not increase the one who pardons except by more honor..." [Muslim and At-Tirmithi]
Nonetheless, some scholars stated that it is enough, in case of backbiting and the like, to seek Allaah's Forgiveness and supplicate for the wronged person if he does not know it. However, the majority of the scholars are of the view that one should seek the pardon of the person he wronged in any case whether that person knows it or not.
Allaah Knows best.