We all fall short of God's expectations because we are all born "sinful" - and we aren't talking about "sinfully" rich deserts, we're talking about the serious side of sin. Sin can be summed up as all the things we say, think, do, and don't do, that fall outside of God's holy will for our lives and end up separating us from God.
Sin was brought into world when Satan lured the first people God created (Adam and Eve) into temptation through their own free will and weakness, breaking the perfect relationship between God and us. From that point on, sin became part of our very existence. Because God also demands perfect obedience, our ultimate punishment became death.
Yet God is a loving God whose will is not for us to live in eternal punishment. That's why He sent His Son, Jesus Christ, to live the perfect life He requires and to become our substitute. Christ never sinned - not even once - and then He took our sin upon Himself and died on the cross, on our behalf. When we believe in Jesus Christ as our Savior, He bears our sin and gives us His forgiveness.
"For God so loved the world that He gave His one and only Son, that whoever believes in Him shall not perish but have eternal life" (John 3:16).