Once, a group of people were traveling on rocks at night. A voice came from the sky that whosoever picks up the rocks will regret and whosoever doesn't pick up the rocks will regret. Now, the people were confused. How can this be? Whether you pick up the rocks or not, you will regret!
Anyway, some people picked up the rocks and some didn't. In the morning when they reached their homes, they saw that the rocks turned into diamonds. Now, those people who didn't pick up the rocks started regretting saying, "If only we had picked up some rocks." Those people who did pick some rocks also started regretting saying, "Why didn't we pick up more rocks?" Both sides ended up regretting.
Dear Muslims, this same incident will take place with us on the Day of Judgment. Those people who spent their time in useless things will start regretting by looking at those who achieved a great position in Paradise by spending their time in the way of Allah (SWT). Now, those who did achieve a position in Paradise will start regretting by looking at those who did more than them and are higher in status. "Alas! If we had only done more to please Allah (SWT)."
Allah (SWT), the Majestic said: "And could you but see when the guilty shall hang down their heads before their Lord: Our Lord! We have seen and we have heard, therefore send us back, we will do good; surely (now) we are certain." Noble Qur'an (32:12)