Imagine God is like the best friend you could ever have. He makes a big promise: "I love you all the time, no matter what, and I'm always here for you." This is like what the Bible says in John 3:16, "For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life." It means God loves us so much that He sent Jesus as our friend and Savior.
But just like you'd be nice to your best friend because you care about them, God hopes you'll be kind, honest, and good. It's not to make God love you more (He already loves you the most!) but to show you love Him back. This is like what the Bible says in Ephesians 2:10, "For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them." It means God made us to do good things, like saying thank you to Him for being so great to us.
So, even though God loves us so much and forgives our mistakes, He also wants us to be the best we can be. It's like giving a big thank you to God by living in a good and kind way. The Bible says in James 2:17, "So also faith by itself if it does not have works, is dead." If we believe in God and are thankful to Him, we'll show it by how we act and treat others.
In short, God's big promise is His never-ending love and forgiveness, and our big thank you is trying our best to do good things and be nice to others, just like we would for a super special friend.