The bible tells us that it is only by grace that we are saved, there is nothing we can do to earn the grace. It just is, like God. There is no need to explain, there could be no explanation as to why we as sinners should be given eternal life in the glory of our Savior. I cannot think of one thing I have done that could possibly earn me the grace that my Father has shown me.
Ephesians 2:8-9 ESV
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.