A man’s evaluation of himself is not reliable because he is not willing to scrutinise his conduct and motive as carefully as he should. Self-examination is something that only the Christian can do properly and even he is making progress in this discipline over the course of the Christian life.
Even the believer makes plans and has many good ideas but may not be free from self-interest. His plans may lead him into many sins of omission. Many inappropriate things may enter in because we do not bring them to the Lord for him to evaluate what we do. God looks at our motives. How much is for me? He may frustrate our plans if we proceed without consulting him, so we must stop: check our plans against his commandments. This is a necessary step in all seeking of guidance: what is my motive for doing this?