Genesis 4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? 7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

Another thing God told Cain was, if you do what is right, you also will be accepted. This shows that, God wasn't judging him, He was simply call his attention to what he needs to do right. The offering of Cain represents the works of the flesh, and this cannot please God, because it is not of faith. 

You cannot be accepted based on your humanistic efforts, it has to be by faith so that it can be of grace. Abel presented blood, no human can produce blood, it is God's gift, so also grace. Anything that represent your effort to earn God's righteousness automatically disqualifies you from His acceptance. 

Romans 4:2 For if Abraham were justified by works, he hath whereof to glory; but not before God. 3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.

Whatever you desire from the Lord, you must come to Him believing and trusting in His grace and mercy. This does not mean that we don't do anything, we simply trust Him that He is able to lead us and make manifest what He has promised us in His word to us. We dont force our will on Him but trust in His love and judgement in all situations.