The great Puritan writer, John Owen, explains the very reason for suffering:
The procuring cause of the death of Christ was sin. He died for sin; he died for our sin; our iniquities were upon him, and were the cause of all the punishment that befell him.
Wherein can we be conformable unto the death of Christ with respect unto sin? We cannot die for sin. Our hope and faith is, in and through him, that we shall never die for sin. No mortal man can be made like unto Christ in suffering for sin. Those that undergo what he underwent, because they were unlike him, must go to hell and be made more unlike him to eternity. Therefore, the apostle tells us that our conformity unto the death of Christ with respect unto sin lies in this—that as he died for sin, so we should die unto sin—that that sin which he died for should die in us. He tells us so, “We are planted together in the likeness of his death” ()—“We are made conformable unto the death of Christ, planted into him, so as to have a likeness to him in his death.” Wherein? “Knowing that our old man is crucified with him,” says he (). It is the crucifixion of the old man, the crucifying of the body of sin, the mortifying of sin, that makes us conformable unto the death of Christ; as to the internal moral cause of it, that procures it. See another apostle tells us, “Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind: for he that hath suffered in the flesh hath ceased from sin; that he no longer should live the rest of his time in the flesh to the lusts of men, but to the will of God” (–2). Here is our conformity to Christ, as he suffered in the flesh—that we should no longer live to our lusts, nor unto the will of man, but unto the will of God.
5 For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his. (ESV)
6 We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin. (ESV)