You believe man is good because it appears he does some good things. But God doesn't look at deeds, but upon the heart. He concerned not with actions, but the motivations behind them.
Let's use a biblical example in Isaiah 58. God, through the prophet, is telling Israel of their sin using the way of their fasting. At first it seems He is complimenting them...verse 2. They seem to seek Him, desire to know His ways, keep His ordinances, and delight to approach Him. Why then is the prophet commanded to cry out against Israel to show them their transgression?...verse 1.
God goes on to show them that their outward attendance to these matters is not what He requires. If they truly had a heart for God and understood His ways, and were keeping His ordinances, their practices would be very different. Not only would they do these things, but in doing them their focus would be God and others, and not themselves.
Anyone viewing their outward behavior would believe that they were doing good. But God looks upon the heart, and saw only evil.
So there are none that are good. Not because men can't do good things. Jesus said as much...ye being evil know how to give good gifts to your children...Matthew 7:11. Men aren't evil because they don't do good things. They are evil continually because they do so from the wrong motivation. Their hearts are evil...Jeremiah 17:9. This is why it is necessary to receive a new heart...Ezekiel 36:26. And this is what God does in salvation.