To answer that we need to go back to the early creation stories. There we have a picture of women and men made in the image of God, living in a world which was free from harm, danger and death. The writer of the Genesis stories, speaks of God walking with human beings in conversation and friendship. Whether or not we choose to interpret this literally, the undisputed message is that we were made to live in relationship with God and equally significantly were not intended to die. This is God's ideal for every human being born into the world.
But there's a problem. God is pure and holy, He cannot be in relationship with those who are not. It might be hard to admit is sometimes, but we all know deep down that we are far from perfect. If we are to live in relationship with God, then we need to be cleaned out, so to speak. God does not ask us to justify, make excuses or even pay for the consequences of our mistakes, He simply says "believe that in Christ, I have dealt with your mistakes; admit you need forgiveness and let me make you right". We can then enter into a living relationship with God, made possible through the presence of His Holy Spirit. Christianity is not about following a whole set of regulations, or turning out for the right services and rituals, it's about allowing God to change us and make us more like Him.
But of course it goes further than that. Many people believe in some form of after-life and if you stop to think about it, it makes sense that if anyone can raise us from the grave, then the one with the power to bring the world into being in the first place can. But the problem returns - how can we live for eternity in God's presence, if we are damaged, messed up beings. This again highlights how important God's offer of forgiveness is - not only can we live in this world in relationship with Him, but we can enter into the next confident, not in being made right through our own efforts and resolve, but through Jesus putting things right on our behalf.