The God of Death chose Sirius because he believed that Sirius was the only one in the Kingdom of Death who could understand his feelings, and now it seems that this is indeed the case.

The tears of death means understanding emotions, having joy, anger, sorrow and joy, and crying when losing a loved one and grieving.

The purpose of exchanging the Unbreakable Flower for Sirius's heart is on the one hand to exercise Sirius and make him have feelings, and on the other hand it is for Death's plan. The Unbreakable Flower has the effect of resisting the power of Death. The purpose of the God of Death is to eliminate the legacy he left in the world and prevent the Kingdom of Death from invading the plight.