The fruit of discontentment is disbelief (13-4)

Resentment causes one to deny the favors of GOD. Eventually perhaps, it could even lead to disbelieving in GOD. If the slave is pleased with his Lord in all circumstances, what follows necessarily is being thankful to Him.
He therefore becomes among the pleased and thankful ones.
But if he is devoid of contentment, he becomes among the resentful ones and follows the ways of the disbelievers.
Falsehood and deviation in beliefs only occurred because many of people wanted to become their own gods, to the point that many of them attempted to dictate to their Lord their wants and desires.