BUT IF they turn away [from thee, O Prophet, know that] We have not sent thee to be their keeper: thou art not bound to do more than deliver the message [entrusted to thee]. And, behold, [such as turn away from Our messages are but impelled by the weakness and inconstancy of human nature:48
thus,] when We give man a taste of Our grace, he is prone to exult in it;49
but if misfortune befalls [any of] them in result of what their own hands have sent forth, then, behold, man shows how bereft he is of all gratitude!50
This interpolation - necessary for a proper understanding of the context - is based on Razi's convincing explanation of how this passage connects with the preceding one. Man is, as a rule, absorbed in a pursuit of material goods and comforts, the achievement of which he identifies with "happiness"; hence, he pays but scant attention to spiritual aims and values, and the more so if he is called upon to abandon his selfish pursuits in favour of the - to him as yet hypothetical - life in the hereafter.
I.e., when God bestows on him a measure of material benefits, man tends to exult in this "success" as such, attributing it exclusively to his own ability and cleverness (cf. the first sentence of 41:50
I.e., instead of remembering his past happiness with gratitude, he calls the very existence of God in question, arguing that if God did really exist, He "could not possibly have permitted" so much misfortune and unhappiness to prevail in the world: a fallacious argument inasmuch as it does not take the reality of the hereafter into account and is, moreover, based on a concept of God in terms of purely human feelings and expectations.