Psalms 51:1a

Have mercy on me, O God,

according to your unfailing love;

according to your great compassion

blot out my transgressions.

(v1, NIV)

David is asking for mercy according to God's love. If God wasn't loving, would he have mercy on us? For what other reason would he have mercy on us? He certainly doesn't need us. He doesn't gain anything from us that he doesn't already have or hasn't given us already. It is because of love that he has mercy on us if he indeed does show mercy to any of us. In addition, his love isn't limited or flaky. It's not like one day it suddenly runs out. In the ESV, the word used is "steadfast", which means fixed or unchanging. If God loves you he will continue to love you and he will have mercy on you according to this unchanging love.