Why did Allah create us, and what is the wisdom behind the existence of Paradise and Hellfire? Was Allah incapable of admitting all His creation into Paradise?

The strange thoughts that cause pain and distress are mere whispers (waswas) and do not reflect belief; they must be dispelled. Allah Almighty creates and does what He wills with perfect wisdom, and He is not to be questioned about what He does. The human intellect is limited and cannot encompass Allah's wisdom. This worldly life is a realm of trial and test, not a realm of recompense and fulfillment. Allah tests His servants to see how they will act. The believer endures with patience and accepts Allah's decree, for with the greatness of the affliction comes the greatness of the reward. It is necessary to strengthen one's relationship with Allah and faith in Him to repel the whispers of Satan.