Cain was a harder worker than Abel. He was the farmer, and even though I’ve never farmed a day in my life, I think it’s safe to assume that it’s harder to produce “fruit of the land” than to work as a shepherd. And even before God rejected his offering, I can imagine his sense of injustice as he worked his ass off in the fields while Abel napped under the old fig tree.
The 19th century rabbi, Netziv, describes this idea: (Cain’s) struggle to make the earth fruitful was more laborious than his brother’s cultivation of the sheep, and the aim of his toil was to yield necessary food, rather than the luxurious products provided by Abel. In Cain’s opinion, then, his adoption of the simple life made him the more pious brother; it was he who merited God’s favor. 
This theme is mirrored in two stories from the Gospel of Luke. Most famously, the parable of the prodigal son  can be seen as an act of injustice where the undeserving received favor while the diligent and loyal did not. Similarly, Luke’s account of the sisters, Mary and Martha, also reflects favoritism towards the lavish sister while again withholding favor for the conscientious sister. In this brief interaction, Martha works tirelessly while Mary reclines at Jesus’ feet. Martha complains that Mary is not doing her share, but Jesus responds, “Martha, Martha, you are worried and upset about many things, but few things are needed—or indeed only one. Mary has chosen what is better, and it will not be taken away from her.” 
It’s hard to determine why “the more pious brother” is rejected. Practically speaking, Netziv describes Cain as failing to understand God’s economy, which accepts more than one form of service. Devotion can come in both strenuous labor and in leisure. It may also point to a Christian theology of God’s preference for “what’s in the heart” over actions and production.
But there’s still something unsatisfying about any explanation. The injustice lingers and never fully sits well. In the end, I don’t find a moral lesson on how to gain God’s favor. It’s more a simple description that life has injustice in it, and injustice is heart-breaking and humiliating and unusually difficult to accept.
This topic is on my mind today as I’m preoccupied by a friend’s recent job loss. At this job, she probably worked as hard as anyone could. Conscientious to the core, I can’t imagine she really did anything wrong. She just wasn’t what they were looking for.
I hurt for her. And I imagine it’s no consolation to recall that history is full of the hardworking faithful that yet could find no favor in the eyes of God.
 The Rabbi Naftali Zvi Yehuda, as summarized by Rabbi Shalom Carmy in his article, “Cain and Abel and the Fairness of God”.
 Luke 15:11-32
 Luke 10:38-42