The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance. But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed. 2 Peter 3:9-10 ESV
If you are a Christian, you will have at least some sense of how important every single soul is to God.
Satan is in a literal life and death struggle with God. Imagine how important each soul is to him.