What did Cain say to Abel anyway?

This story begins with Eve giving birth Cain, quickly followed by the birth of Abel. Little is said about the two brothers-- we are not given much insight into their personalities. All we are told is that one became a farmer and one became a shepherd. At some point, both brothers bring an offering to God. This may have been part of a ritual, or the boys may have come to the decision to bring offerings to God independently. They boys bring from what they have to give-- Cain offering the fruit of his labor and Abel offering the best firstling of his flock. Tradition holds that Cain's sacrifice was lesser than Abel's in some way-- perhaps he gave what he had left over rather than the best of his grain. Perhaps he gave from his excess while Abel gave what was most precious. The text on this matter is silent. We are only told that God noticed, or prefered, Abel's offering, while He ignored, or rejected, Cain's gift.
God does take notice, however, of Cain's reaction to divine rejection-- and He is surprisingly puzzled by it. He actually chastises Cain for feeling badly about going to the effort of preparing an offering, only to have said effort be ignored. Cain's disappointment suggests to me that he had done his best to present an offering pleasing to the Lord. It strikes me that one might not be disappointed if one had put together a half-assed sacrifice, because one could tell himself that he could put for a better effort in the future. I really think Cain did the best he could and it simply wasn't good enough. And that is a disappointing moment in anyone's life-- the moment one realizes that whatever he or she has to offer will never be good enough and will never be acceptable.
And God cannot even bring himself to acknowledge Cain's pain as legitimate. He basically tells him to cheer and that if he does well, he will be rewarded, suggesting that whatever effort Cain put into his harvest offering was not right, because the reward was silence. It seems to me that God is simply telling Cain that He prefers Abel and Cain should just get over it. And how can Cain bring his pain to God-- now that God refuses to recognize his grief.
And in my modern perspective, I have to wonder why God prefers the bloody sacrifices. Abel had to kill his offering and that was pleasing to the Lord. Much more so than the offering of bloodless grains. What is it about blood that God seems to require in gifts and sacrifices?
So Cain speaks to his brother, and what is said has been lost forever. The end of the conversation is Abel's death. I've read legends that Cain, having realized he failed to give God what he loved the most, determined to give his baby brother out his love for his brother-- giving up what was most precious to him. They are interesting legends, but I do think Cain probably killed his brother out of anger and jealousy. Out of his feelings of being replaced.
Perhaps he should have spoken sooner. After his brother's murder, he does argue with God-- protesting his punishment. And I have to wonder, if Cain had protested the favoritism, would he have vented his anger before rising against his brother? Why does the taking of a life suddenly render him brave enough to argue with God?
And out of his exile, civilization comes into being. He founds a city. His descendents create music and technology. Yet years later, he is killed by one of his grandchildren in a moment of impulsive violence. Maybe that is the lesson-- that we will always compete. and only some will make the grade, and we have control our reactions to our failures, even when we're not sure why we failed.