God is "rich in mercy" (Eph 2:4). He delights "in showing mercy" (Mi 7:18, JB). "His mercy endures forever" (Ps 136:1). God desires mercy (Mt 9:13).
Fr. Lauer, publisher of One Bread, One Body, has long defined mercy as treating someone better than they deserve. To treat someone as they deserve is to give them their due. This is a fair and just cost to pay, and "even sinners do as much" (Lk 6:33).
At a higher level than paying someone their due is to sacrifice for someone. This involves paying a higher cost to serving someone, going above and beyond the minimum. Sacrifices come at a personal cost. They require time, energy, love, and forethought. "God is pleased by sacrifices of that kind" (Heb 13:16).
However, ultimately it is not sacrifice that God desires; His heart is set upon mercy (Mt 9:13). Jesus' disciples are set apart by treating others better than they deserve, including enemies, persecutors, criminals, sinners, etc. Jesus' disciples are "impelled by love" (see 2 Cor 5:14) to imitate their Master by laying down their lives for people who don't even care about them (see Rm 5:8), regardless of whether or not that mercy is even appreciated. "This will prove that you are" children of God (Mt 5:45), rich in mercy (Eph 2:4). This is what God desires.