Rashi: Even an Aveirah that could be forgotten, it is remembered via adding Aveiros. The latter hatred arouses [previous sins. Yechezkel rebuked Yisrael for Aveiros [like those of] Egypt - "va'Omer Alehem Ish Shikutzei Einav Hashlichu [uv'Gilulei Mitzrayim Al Titama'u]" (20:7). This hatred was suppressed for many years. Hashem did not mention it until now, that they added Aveiros to their Pesha'im.

Malbim: Medanim is unlike Riv. Riv is quarreling with another, sometimes over nothing; the matter they quarrel about is called Merivah. If it is due to money or affliction, for which he can take his opponent to a judge, this is Madon. It is not clear who is justified. If it is clear, this is called Peshi'ah, Begidah, or Mishpat, according to the case. The author teaches, the reason for these Medanim is not the disputed matter itself, rather, agitation in the parties' Nefashos. Lovers, or people without hatred, are not adamant about small matters like damage of neighbors. Hatred arouses the quarrels. It makes them stingy and seek the other's evil. This awakens quarrels that would have slept, if not for the hatred.

Rashi: When Yisrael improve their ways, Hashem covers over their Pesha'im.

R. Yonah: If one loves his friend, even if the latter sinned against him, he does not quarrel with him. He covers (overlooks) his sin due to his love. Therefore, one must beware of hatred, which leads to anger and quarrels. He should also avoid all dealings with a hater, and choose to deal with lovers.

Ma'alos ha'Torah: Hashem's entire Simchah is in Torah. Love of Torah covers all a person's sins.

Malbim: Love covers even great trespasses, and all the more so they will not arouse quarrels over minor matters.