Being happy in ones suffering through constantly remembering ones sins:
The verse states “Vechatasi Lenegdi Samid”, that one’s sins are to be constantly before him. One’s constant reminder of sins allow him to accept suffering that he experiences, and allows him to rejoice in his suffering, knowing the suffering refines him from sin. This joy in one’s suffering brings one to complete atonement.
 Tanya Igeres HaTeshuvah chapter 11