Do you think you are a good person? 

The bible say's every person is pure in their own eyes and every person will proclaim their own goodness. -Proverbs 30:12/Proverbs 20:6

People do this because they have no idea about the righteousness of God. They have no standard to compare themselves to.

A little girl was looking at sheep eating grass. She thought how nice and white the sheep was against the green grass. 

Then it began to snow. The girl looked back at the sheep and thought how dirty the sheep now looked against the white snow.

Same sheep. Different background. When you and I compare ourselves to the background of man's standard, we come out reasonably clean. 

There are plenty of people worst then me. Adolph Hitler makes me look pure white. But when we have the snowy white righteousness of God's law as the backdrop, suddenly we can see ourselves in truth. That's the purpose of the law.

The law of the Lord is perfect, converting the soul. -Psalm 19:7