"Whoever works righteousness, whether male or female, while he (or she) is a true believer (of Islamic monotheism) verily, to him We shall give a good life (in this world with respect, contentment and lawful provision), and We shall pay them certainly a reward in proportion to the best of what they used to do (i.e. Paradise in the Hereafter)." (Soorah an-Nahl, 16:97)
Shaykh Muhammad Ibn Salih al-'Uthaymeen رحمه الله comments,
"What is the 'good life'? Is it abundance in wealth? Is it having many children? Or is it having security in one's life? To be sure, the 'good life' is having a cheerful, peaceful, and contented heart, even if one is in the most difficult of circumstances. The Prophet صلى الله عليه وسلم said, 'How wonderful is the affair of the believer, for the affair of the believer is good in its entirety, and that is [a quality] that is for no one save the believer. If he is afflicted with hardship, he is patient, and that is better for him. And if something good befalls him, he is thankful [to Allah], and that is better for him.' 
When a disbeliever is afflicted with hardship, is he truly patient? No, rather, he becomes sad and the world becomes constricted for him, and perhaps, he may even end up taking his own life. But the believer is patient and finds that the joy of patience results in tranquility and contentment. That is why his life is a good life. Hence, the saying of Allah: 'We will give [them] a good life.' is referring to life in one's heart and self.
Historians mention an interesting incident in the life of al-Hafidh Ibn Hajar - may Allah have mercy on him. He was the Judge of Egypt during his era, and when he would go to his place of work, he would arrive in a carriage that was drawn by horses or mules in a procession. One day, he passed by a jew who was a seller or oil, and it was common for a seller of oil to wear dirty garments. The jew halted the process and said to Ibn Hajr (may Allah have mercy on him, 'Indeed your Prophet says: "The world is a prison for the believer and paradise for the disbeliever." Yet you are the judge of judges in Egypt, and you are in this procession, in ease and comfort. And here I am, in this state of punishment and misery!'
Al-Hafidh Ibn Hajar (may Allah have mercy on him) said, 'If the situation I am in is really a state of comfort and ease, then it is a state that is prison when compared to the ease (and comfort) of Paradise. And the misery you are in is Paradise when compared to the punishment of the Hellfire.'
The jew said, 'I bear witness that none has the right to be worshipped but Allah, and I bear witness that Muhammad is the Messenger of Allah.' And thus he accepted Islam.
So no matter what the situation, the believer is in a good state, for he gains profit in this world and in the Hereafter. On the other hand, the state of the disbeliever is truly evil, for he has lost out on all good in this world and in the Hereafter (i.e. even if he is prosperous in the material sense, he will not achieve the contentment and tranquility of the believer in this world; and as to the Hereafter, his eternal above is the Hellfire).
Allah سبحانه و تعالى said:
"By Al-'Asr (the time). Verily! Man is in loss. Except those who believe and do righteous good deeds, and recommend one another to the truth, and recommend one another to patience." (Soorah al-'Asr, 103:1-3)
The disbelievers have forsaken Allah's Religion and have lost themselves in their lusts, desires, and excesses. Even if one of them builds castles and is very affluent in the world, in reality he is in Hell. Some of our pious predecessors have said, 'Were the kings and the sons of kings to know what [the state in which] we are in, they would fight us over it with swords.'
The believers find joy and delight in invoking Allah سبحانه و تعالى and remembering Him, and they are always content with Allah's Decree and Divine Preordainment. If they are afflicted with hardship, they are patient. And if some blessing befalls them, they are thankful. Hence, they are in a state of delight, as opposed to worldly people, whom Allah سبحانه و تعالى described with the following: "If they are given a part thereof, they are pleased, but if they are not given thereof, behold! They are enraged!" (Soorah at-Tawbah, 9:58) 
 Saheeh Muslim
 The Islamic Awakening, pp. 24-25