Why was Cain jealous of Abel?

Answer 1

Abel worshiped God according to the instructions provided,whereas Cain worshiped God his own way.

Mk:7:7: Howbeit in vain do they worship me, teaching for doctrines the commandments of men.

Answer 2

God favored Abel's sacrifice over Cain's because Abel's was a sacrifice made from love of his 'flock' while Cain's was a result of his own works. It isn't stated nor believed that God rejected Cains sacrifice but it was Cains presentation and actions thereafter borne of jealously that got him punished. Abel was a good man, Cain was described as prideful, which is not a good thing in God's eyes.

* I think also a lamb was the required/acceptable sacrifice.

This is what Mathew Henry's commenatry says:

2. There was a difference in the offerings they brought. It is expressly said (Heb_11:4), Abel's was a more excellent sacrifice than Cain's: either (1.) In the nature of it. Cain's was only a sacrifice of acknowledgment offered to the Creator; the meat-offerings of the fruit of the ground were no more, and, for aught I know, they might be offered in innocency. But Abel brought a sacrifice of atonement, the blood whereof was shed in order to remission, thereby owning himself a sinner, deprecating God's wrath, and imploring his favour in a Mediator. Or, (2.) In the qualities of the offering. Cain brought of the fruit of the ground, any thing that came next to hand, what he had not occasion for himself or what was not marketable. But Abel was curious in the choice of his offering: not the lame, nor the lean, nor the refuse, but the firstlings of the flock - the best he had, and the fat thereof - the best of those best.

Answer 3

God gave us some choices to make on our own but that doesn't mean that God didn't love Cain any less then Abel.