Sin is the transgression of God's laws. (1 John 3:4) Our God made moral laws, as well as all the laws that govern all of his creation, like the laws of nature, of the universe, etc. Just as concrete and precise and whole the laws of nature are, so God's moral laws must also be perfect. Therefore we can't pick and choose for example the ten commandments, ALL are equally important (though we may not agree or understand it). We are all sinners because there's no way to never transgress on His laws. Sin is the greatest problem because it keeps our God and us apart - because He is HOLY and sin cannot touch what is Holy.