First of all, what kind of ingrate are you? If someone gave you a billion dollars, would you not even say “thank you” to your benefactor? A billion dollars is less than a pittance compared to eternal life! Trust and obedience are the most appropriate ways to thank God for His gift of never-ending existence. Since life on earth will therefore be – at most – an ever-decreasing fraction of our total existence, wouldn’t it be entirely appropriate for us to spend the remainder of it serving the One who suffered on a cross so that it could become that ever-decreasing fraction of life instead of the totality of it?
Secondly, just because you know that everyone is going to heaven doesn’t mean that you know where in heaven everyone goes. It’s a really big place! Given the quantity and variety of locations we find on earth, the quantity and variety of locations in heaven must be far, far greater. God has promised that we will be rewarded in heaven based on how well, or how poorly, we performed on earth; if there are places on earth you’d rather be, how much more there’ll be places in heaven we’d rather be. Further to this point, we often see the judgments of God – for good or evil – executed on earth, but just as often we are confused and troubled when it seems that injustice prevails. The justice we experience on earth is limited, but heaven is God’s opportunity to right all wrongs and render each and every person the granular justice deserved. That we get to heaven at all is undeserved, but exactly where we land in heaven will be exactly the place we deserve.
Thirdly, trusting and obeying God is necessary if we are to avoid the fires of hell…which are on this earth in this life. Hell is a result of sin and God’s judgments on it. Thus, when the sins of mankind increase, suffering in the world increases. Therefore, we should at all times seek to do right so that our behavior brings blessings instead of curses both to ourselves and to others. To sum up, even though everyone is going to heaven, we should still trust and obey God because 1) showing gratitude is the right thing to do, 2) we’ll want as good a placement in heaven as we can get, and 3) we’ll only make our time on earth harder for ourselves and others if we continue to sin.