First of all with all these blogs I don't want anyone to think that I'm setting myself up as a perfect sinless person. I'm not. Not by a long shot.
With that out of the way I'll get to the point! :)
I've derived this from something Paul Washer once said. To understand why sin is a big deal to God, even small sins one must understand the magnatude of sin.
God commanded the universe to exist out of nothingness. It obeyed him.
God commanded entire gallaxies to go where they should, and they went.
God commanded enormous stars where to hang in the heavens and they obeyed him.
God commanded mountains to rise and valleys to fall and they obeyed.
God commanded the ocean to stop and allow the dry land to appear. It obeyed him.
Then, God turned to man and said come to me and man said "NO!".
Mankind is the only creation that has ever disobeyed the word of God, the same word that created the universe. That is a big deal!