God is good. He isn’t this way because He has to be. God is good because that is His nature – that is who He is as a person. It flows from Him. He doesn’t just do good because it is good to do good, He is good.
We humans are not good. It is our nature to be evil and to do wrong. Our nature is diametrically opposed to God’s nature.
But it’s natural, right? If it’s our nature, surely we can’t help it and so we are given excuse.
But we can help it. Not by ourselves, but we do have a choice as to what nature we have. God is good, and He is powerful enough to change our nature to be like His. So we have the ability to go to Him and be changed!
When a person repents of their sin and is saved by God, they are given a new nature, a new heart, new desires. But these don’t show up lived out in your life all at once, and they certainly aren’t easy to live out. We still struggle with our sin nature, the part of us that wants to do wrong things, and wants to do things our way without worrying about God or other people or consequences to our actions (Romans 7). One day when Jesus returns that struggle will be done away with, but for now we fight battles that God wins for us. God has the strength to give us so that we can live according to our new nature right now!
At first, we do things that are good because we know it’s right. We should obey and we should do right because it is good and right. But as God works in our new hearts, we begin to do good because that’s who we are. We aren’t ever perfect, but it gets easier to do right without fighting about it. Sometimes you may not fight the good at all. God is changing who you are. You’re still you, of course, but your nature is changing (Romans 12:2). You want more of what God wants. You love more of who God loves. You can be a partaker of God’s nature (2 Peter 1:3-4).