Who said you have to forgive those who don’t want you to?
The Ramban seems to say it’s midas chassidus (depending on what you mean by “don’t want you to”).
?? ???? ???? ????? ??? ??? ??????? ??? ???? ?? ???? ???? ???? ????? ????? ?????. ?? ????? ???? ??????. ???? ?? ???? ??? ???? ??? ?????? ??? ?? ??? ?????? ?? ?????? ???? ??? ?? ??????
http://www.hebrewbooks.org/rambam.aspx?rid=150
The reason, simply, is because the Torah doesn’t want us to carry sinah in our hearts.
The meforshim say different pshatim in this difficult Rambam, but that is the simple understanding.