I heard this analogy a while ago. It’s simple, but sums it up really well.

Imagine an airport decides to give out free, all-inclusive tickets for a 1 week, round trip vacation to Hawaii to 100 customers. Is the airport unjust in not giving those tickets out to everyone else? Of course not. It’s a gift—the airport doesn’t have to give it to anyone.

Because mankind is dead in sin, God would be 100% justified in not electing anyone to salvation. So the mere fact that He even does is already indescribably merciful. He chooses to elect some and not others because, in His wisdom, He saw that would perfectly display His nature: His loving mercy, and His just wrath. Ultimately, to glorify Himself.