"Why is church so negative?"
It is easy to look at a doctrine like original sin and think that it is just flat out pessimistic. "Sins are bad things, right? Like stealing and murder and the whole Ten Commandments thing, right?" When we talk about sin, we must begin with God. What does God desire for us? God desires us to flourish in love and hope and faith. God desires to live in relationships of mutual uplift. Sin is when we do not do this, when we act against God's will and plan and hope for us of abundant life. Sins are not just things our parents tell us not to do. Sins are inherently destructive because they attack the image of God in us, they deny who we were created to be. Even when we do not see the direct consequences of our actions, sins harm us.
Let us imagine a lie. You forget to respond to a phone call from a friend and instead of admitting it, you tell them you never got it. Your friend may never realize that you lied. This does not change the effect that the your lie has on you in taking you away from the truth of God's love and plan for you. Every lie is an absence of faith.
But, eek, what about all the little sins that we do every day. Wouldn't it be absolutely overwhelming to have to remember about all of those and to feel sorry for all of those?
This is where we get to the grace that is found in understanding original sin. Original sin teaches us that every person tends to corrupt what they have. That at its basest level, we are not to blame for where we start. We should not be surprised by our tendency towards sin. We should not be surprised by the brokenness of the world.
This is the miracle of grace. This is the miracle of the coming of Jesus Christ. We did not deserve grace but God loved us so much that he gave his only begotten son that whosoever believees in him will not perish but have everlasting life.
As well, original sin reminds us that we cannot save ourselves. No matter how hard we work or how hard we try, apart from God's grace, we will not flourish.
As Christians, we should be able to honestly look at our sins and not be ashamed because of the grace we have been given. Our sins don't point to our weakness. It is when we hide, when we pretend we are people that we are not that we are truly week. When we admit our need for grace, our need for God, when we seek, we shall find mercy and the true freedom to be who we were created to be.
Article VII - Of Original or Birth Sin
Original sin standeth not in the following of Adam (as the Pelagians do vainly talk), but it is the corruption of the nature of every man, that naturally is engendered of the offspring of Adam, whereby man is very far gone from original righteousness, and of his own nature inclined to evil, and that continually.