Cain's offering

Gen 4 v3-7
Cain brought an offering of the fruit of the ground and Able brought an offering of the firstlings of his flock.
It says God was pleased with Ables offering but not with Cain's offering. Have you ever wondered why God was pleased with one and not the other. Well the first thing you might think is that God had killed an animal when Adam and Eve sinned therefore God wanted animal sacrifices. Well yes we know that from other scriptures that without the shedding of blood there is no forgiveness for sin, but it does not say that Cain and Able were offering a sin offering.
It says about Able that he brought the firstlings of his flock. God is interested in the attitude of the giver. Able brought what he got first and Cain just brought some of the fruit of the ground.

In verse 7 God speaks to Cain and says if you do well you will be accepted. Is this still not true today?
2Ti 2:15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.