Quran

This is worse than merely staying behind, because staying behind was prohibited, but what made it worse was being pleased with having done an act of disobedience, and rejoicing in it, (and disliked to strive with their wealth and their lives in the cause of Allah). This is in contrast to the believers who, if they stayed behind -even if that was with a valid excuse- would feel sad and be filled with sorrow for staying behind, and they would love to strive, offering their wealth and their lives in Allah's cause because of the faith in their hearts and of what they hoped for of the bounty, kindness and blessing of Allah. Al-Saadi: 346.
Question: What is the difference in the reaction of a believer and that of a hypocrite when they miss good deeds?

This verse describes their situation as a rebuke to them that implies a threat. (Those who were left behind) indicates a disdain for them, and that they were left out of Allah's pleasure. The use of the term, (were left behind) is more suitable in this case than the term, "stayed behind". Rejoicing, in this case, was only done by a hypocrite, and therefore, the three Companions who remained behind and those with excuses were exonerated. Ibn Atiyah: 3/65.
Question: Allah says, (were left behind), not "stayed behind", why. What do we benefit from that?

Thus, they gave precedence to a brief and temporary comfort over a complete and eternal comfort. They were afraid of the heat from which one may protect himself by seeking shade, and which disappears in the early morning and late afternoon, yet they did not fear the intense heat, the severity of which cannot be estimated, namely the fire of Hell. Al-Saadi: 346.
Question: What is reason for describing the hypocrites as unable to understand?

The Companions were used to laugh, but frequent and habitual laughter is censured. It is normally associated with fools and idlers. It is said that frequent laughter causes the heart to be dead (indifferent). Al-Qurtubi: 10/318.
Question: Explain how should the believer manage laughter.

Any one who is reluctant and decline to participate in doing good deed that has been enjoined when the opportunity arises to do it, will not be helped after that, and will be deprived of doing it on subsequent occasions. Al-Saadi: 346.
Question: What is the risk of declining to participate in doing good deed when the right opportunity comes?

The proper act of Sunnah when visiting Muslims' graves is invoking Allah to have mercy upon them, which is similar to performing funeral prayer before the burial. Allah says, (Never offer the funeral prayer for any of them who dies, nor stand by his grave). So, it is inferred from this that invoking Allah to have mercy upon the dead believers is ordained. Ibn Taimyah: 3/435.
Question: What is Sharia ruling on praying for the believers at their gravesites?

This is a lesson for believers that liking and disliking should be for the sake of Allah, as it is one of the most subtle and important concepts in Islamic faith. It is a concept upon which most subject matters in Islam are built, and from which most benefits and etiquettes are attained. That is because when one sees another who has been given worldly gains and thinks that he is among those who will be saved, then, he becomes affectionate toward him for his good speech while being heedless of his bad actions. Al-Biqai: 3/371.
Question: How do we infer from this verse the belief of allegiance and renunciation?