How can one who enters Paradise be happy while one of his family or loved ones is in Hellfire?

Measuring worldly matters by the standards of the Hereafter is incorrect, for each abode has what is appropriate for it. The people of the Hereafter are preoccupied with themselves, and the people of Paradise have no fear, nor do they grieve, nor do they die, nor do they age. Allah has removed grief from the people of Paradise for any reason, whether through forgetfulness, or by casting the hatred of the people of Hell into their hearts, in contentment with Allah's wisdom and justice, or by transforming the disbelievers into forms that repel the people of Paradise.