ELI5: If you're a Christian, why do you have to be good if Jesus will forgive you no matter what?

Becoming a Christian isn't about having to be good in order to avoid punishment, it is instead about being given the freedom to do the good you would like to do but find yourself not able to do.

Most people have the experience of having standards for themselves they don't live up to. Jesus was the perfect person you want to be but can't be, and died to give you the opportunity to know God like He did/does so you can actually start to become that person.

You're right that he forgives no matter what. That's because he loves no matter what. Image you saw a child living on the streets alone, and you wanted to adopt them. You then pay hundreds of thousands in adoption fees, supplies, etc. In order to bring them into your family because you care about them and love them so much. It wouldn't matter how many times they continued to act out, their behavior was not the point. Rescuing them from a cold dark world was the point. And the more time they spend in your loving family, the less and less they will feel the need to act out because they learn that it isn't necessary anymore. No more stealing, your food is free. No more fighting, dad looks our for me now. No more getting sick for weeks, doctors visit the next day.

“Come to me, all of you who are struggling and burdened, and I will give you rest. Take my yoke upon you and learn from me, because I am gentle and humble in heart, and you will find rest for your souls. For my yoke is easy, and my burden is light.” Matthew 11:28‭-‬30

“Therefore, I tell you, don’t worry about your life — what you will eat or drink; or about your body — what you will wear. Isn’t life more than food and the body more than clothing? Look at the birds flying about! They neither plant nor harvest, nor do they gather food into barns; yet your heavenly Father feeds them. Aren’t you worth more than they are? Can any of you by worrying add a single hour to his life? “And why be anxious about clothing? Think about the fields of wild irises, and how they grow. They neither work nor spin thread, yet I tell you that not even Shlomo in all his glory was clothed as beautifully as one of these. If this is how God clothes grass in the field — which is here today and gone tomorrow, thrown in an oven — won’t he much more clothe you? What little trust you have! “So don’t be anxious, asking, ‘What will we eat?,’ ‘What will we drink?’ or ‘How will we be clothed?’ For it is the pagans who set their hearts on all these things. Your heavenly Father knows you need them all. But seek first his Kingdom and his righteousness, and all these things will be given to you as well. Don’t worry about tomorrow — tomorrow will worry about itself! Today has enough worry already! Matthew 6:25‭-‬34

And eternal life is this: to know you, the one true God, and him whom you sent, Jesus the Messiah. John 17:3

Don’t you see how wonderfully kind, tolerant, and patient God is with you? Does this mean nothing to you? Can’t you see that his kindness is intended to turn you from your wrongdoing? Romans 2:4