To begin with, all suffering is a consequence of sin–either from our own sins or from the sins of others. The Son of God became flesh so that, by becoming one with us in the flesh, he could experience the consequences of each of our sins…and Love us until the end despite those sins. By loving us despite our sins we are placed in the same situation as the adulteress in Scripture. Jesus tells those who wish to stone her that he who is without sin can cast the first stone. When all the “accusers” leave, Jesus tells her that since she has no accusers, he will not accuse her. By dying for our sins (he experienced the consequences of every sin that every one of us would commit) he took away ALL of our accusers–except ourselves if we don’t avail ourselves of his grace.
When we forgive others who sin against us, we do our part to remove ourselves as an accuser of someone God loves so much that his Son died for their redemption and salvation. Furthermore, God allows suffering because from it He accomplishes good even though we are usually not able to fathom the reasons. When I look back on my life I can say that I brought the majority of my suffering on myself through sinful choices. Of that suffering that was not self-inflicted, I am usually able to see the good that was accomplished through it.