Man is fickle when Ease comes to Him after Difficulty
Allah tells us that man never gets bored of asking his Lord for good things, such as wealth, physical health, etc., but if evil touches him -- i.e., trials and difficulties or poverty --
فَيَئُوسٌ قَنُوطٌ
(then he gives up all hope and is lost in despair.), i.e., he thinks that he will never experience anything good again.
(And truly, if We give him a taste of mercy from Us, after some adversity has touched him, he is sure to say: "This is due to my (merit)...") means, if something good happens to him or some provision comes to him after a period of difficulty, he says, `this is because of me, because I deserve this from my Lord.'
فَذُو دُعَاءٍ عَرِيضٍ (he is full of broad prayers - 51.) The nature of an infidel is being stated here that when Allah Almighty bestows upon him the good things: wealth, honor and peace, the infidel gets so involved in them and so intoxicated with them that he becomes even more distant from the true Benefactor, Allah Almighty, and his pride and heedlessness increase even more. But when he faces some distress, then he starts long and lengthy supplications to Allah Almighty. Here the Qur'an has used the word ` broad' to describe their lengthy supplications, because what is wider will automatically be longer too. That is why, when describing the span of Paradise, Allah Almighty has used the word عَرْضُهَا السَّمَاوَاتُ وَالْأَرْضُ (a paradise the width of which spans the heavens and the earth - 3: 133)
Lengthy supplications in themselves are praiseworthy and desirable as is proved by authentic ahadith about the etiquettes of supplication where it is mentioned that, while praying to Allah Ta’ ala, beseeching tearfully and repeatedly is desirable. (Bukhari and Muslim) Therefore, the denunciation of the infidel is actually not for lengthy supplications, but for his mean disposition that when Allah blesses him with His bounties, he gets intoxicated with arrogance and pride, and when he faces distress, he keeps on praying in a way of wailing and complaining about his difficulties, as is the habit of heedless people; the intent is not to supplicate, but to wail about his misfortunes and to keep on telling the people about it. (Allah knows best).
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 50–51
The moment of crisis for a man is the moment of self-discovery. So, when he is confronted with adversity, he forgets self-conceit and starts remembering God. At that time, he realises that he is the subject or servant of God and God is his Deity. But, when God gives him relief from his trouble and blesses him with amenities, he immediately forgets his earlier position. He links the grace of God bestowed upon him with the cause-and-effect chain and considers it to be result of his own devices and abilities. He develops a perverse mentality according to which he thinks that the only life is that of this world, and that there will be no reckoning in the Court of God. Furthermore, his prosperity leads him into the misunderstanding that since he is well-off here, he will definitely be well-off in the hereafter also.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.