If you, O LORD, should mark iniquities, O Lord, who could stand?
But with you there is forgiveness, that you may be feared.
(Psalm 130:3-4 ESV)
The greatest thing in all the world is to know God and to live in peace and fellowship with Him. That is why we exist. What then keeps most people from experiencing this kind of life? The short answer is our sin:

Behold, the LORD's hand is not shortened, That it cannot save;
Nor His ear heavy, That it cannot hear.
But your iniquities have separated you from your God;
And your sins have hidden His face from you,
So that He will not hear. Isaiah 59:1-2
It follows then that sin is our biggest problem, whether we realize it or not. Other things may cause us much pain and trouble, but only our sin can separate us from God and expose us to eternal wrath. But there is forgiveness available with God and when we really see how sinful we are, how holy, righteous, and good God is, and what God has done that He might be able to justly forgive us and bring us into fellowship with Himself, then we really should humbly receive that forgiveness and then live joyful lives in reverence and awe of God and in fellowship with Him and in dependence upon Him.