Abel was a keeper of sheep, but Cain was a tiller of the ground.
In process of time it came to pass, that Cain brought of the fruit of the ground an offering to the Lord.
Abel also brought of the firstlings of his flock and of the fat of it.
The Lord had respect for Abel and for his offering; but for Cain and for his offering he did not have respect. (Leviticus 4:3-5)
“from the fruit of the ground”-It was poor in quality. But we have a tradition stating that it was linen.(Rashi)
Some say that Rashi quotes the tradition stating that Cain brought linen, which is considered by Torah to be a most prestigious specie of the vegetable kingdom, in order to explain how Cain thought his offering would be well accepted notwithstanding it’s being of poor quality.
Cain thought, for spiritual reasons that I may post later, that the quality of the specie being offered was the only critical factor governing the acceptability of an offering to G-d; he didn’t give much weight to the quality of the specimen itself.
Whereas Abel thought that what mattered most was the quality of the specimen, rather than the quality of the specie constituting his offering. So while Abel only brought sheep, which is considered by Torah rather less prestigious than cattle, he brought “the firstlings of his flock” and “the fat of it.”
We learn from G-d’s reaction that Abel was correct.
This idea is found as well in Rambam : One who wishes to be meritorious should force himself and bring [to G-d] from the very best of whatever specie he happens to be offering.
The Rambam implies that one should choose the best specimen of the specie he is offering to G-d, but he need not necessarily bring of the best specie. There is ample reason to suggest that he learned this from our episode.
The reasoning behind the idea is thus: the purpose of Torah’s directive that “all the fat(best) is G-d’s” is to express Man’s recognition in G-d’s ownership over the whole creation: “The whole world and everything in it is G-d’s.” (Psalms 24:1)
And since “the world and everything in it” includes both prestigious species and not, the concept of “all the fat (best) is G-d’s.” needs to be expressed in all species in the creation. Therefore, one needs always to bring “from the very best of whatever specie he happens to be offering.”
(Adapted from the teachings of Rabbi Menachem Mendel Shneerson.)