Well, the Orthodox would simply say "because He loves us more than we could ever love Him."
But, more importantly, how do we know He has mercy on us? When we pray, we simply ask for mercy. We don't know, but we hope and believe in His love. "Now faith is the substance of things hoped for, the evidence of things not seen" (Heb 11:1)
I do not understand what you mean by it being a commandment
God chose Moses to do a formidable task. Christ chose the Apostles. There were many chosen God's people who accomplished specific tasks as God's elect. The Jews were chosen to spread the knowledge of God, so that "salvation is from the Jews." (Jn 4:22) And, as Christians, we are to serve as the light of God for the world to see and glorify Him. (cf Mat 5:16)
It's just a guess, mind you, but could it be...'cause the Bible tells me so???
On second thought, it's not a guess at all. I believe the Bible because it is God's will that I believe the Bible and that He has given me the Holy Spirit to guide my every step. As He's given to all those who possess Trinitarian faith in Jesus Christ.
We never knew love until He loved us.
We do know if we trust God to keep His word...
1Pe 1:1 Peter, an apostle of Jesus Christ, To those who reside as aliens, scattered throughout Pontus, Galatia, Cappadocia, Asia, and Bithynia, who are chosen