Today we talked about Cain and Abel's offering... Why was Abel's offering more acceptable? I thought it might have had something to do with the fact that Abel was offering up what God had given him and Cain was offering up something he had worked for; but that didn't quite make sense to me. Why would God not accept something just because the person had worked for it? This makes it seem that no matter what Cain offered, it could never be acceptable because he was not a keeper of the fields.
There is more to it. The passage says specifically that Abel offered the best of what he had. It doesn't say this about Cain. In fact, Cain's actions afterward show that he felt that God should be obligated to accept him. This attitude was probably in his heart before the offering, perhaps coming from the knowledge that his family had been driven away from God's presence in Eden. Now not only was he denied Eden, but he was denied closeness with God because his offering was not good enough. So maybe if he killed Abel, then God would have to accept him because Abel wouldn't be in the way.
Of course, Abel was never the problem. Cain offered his fruit out of his own sense of obligation, not out of love for God. God doesn't want obligatory sacrifices. They are an insult to His nature as a loving, relational being! On the other hand, true worship is not possible without obedience and sacrifice.