Why does Allah (swt) delays the answer of prayer from his believers?

“Once I said to abu al-Hassan (al-Rida) (as), ‘May Allah keep my soul in service for your cause, I have been praying to Allah for help in something from the year so and so and because of delay something is happing in my heart.’
The Imam said, ‘O Ahmad, beware of Satan! Do not allow him to find a way in you to despair you. Abu Ja’far (as) would say, “A believing person prays and pleads before Allah, the Most Majestic, the Most Holy, for help and help is delayed due to love for his voice and listening to his intense weeping.”’
The Imam then said, ‘By Allah, whatever Allah, the Most Majestic, the Most Holy, delays for believing people of the help that they may ask in this world is far better for them than helping them quickly. Such a worthless world! Abu Ja’far (as) would say, ‘It is very proper for a believing person to pray for help in his comfort just like in hardships. He should not grow to be lazy after receiving help to lose interest in prayer.’ You should never lose interest in prayer; it is of a very important position before Allah, the Most Majestic, the Most Holy. You should exercise patience and seek to find lawful sustenance, maintain good relations with relatives and should never make it public. We are a family who maintains good relations with those who cut off from us and do favors to those who cause us trouble. In this we find good consequences. If an affluent person in this world prays and asks for favors, his prayer receives acceptance. He may ask for more and the bounties seem insignificant to him. He does not feel satisfied with anything. When bounties are plentiful a Muslim is in a danger of violating the rights he owes and the mischief that may threaten him. Tell me about yourself. If I will say something to you, will you trust me?’
I (the narrator) said, ‘May Allah keep my soul in service for your cause, if I do not trust you, then whom will I trust? You possess authority from Allah over His creatures.’
The Imam said, ‘Your trust in Allah must be stronger; you have a covenant with Allah. Has Allah, the Most Majestic, the Most Holy, not said, “(Muhammad), if any of My servants ask you about Me, tell them that the Lord says, ‘I am near; I accept the prayers of those who pray.’ Allow My servants answer My call and believe in Me so that perhaps they may know the right direction.’ (2:186) ‘Do not despair of the mercy of Allah. Allah certainly forgives all sins. He is All-Forgiving and All-Merciful.’ (39:53) ‘Allah promises you forgiveness and favors. Allah is Munificent and All-Knowing.’ (2:268) You must trust Allah, the Most Majestic, the Most Holy, more firmly than others. Do not allow anything to find a way in you except good; you are forgiven.’”

Sanad: Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad ibn ‘Isa from Ahmad ibn Muhammad ibn abu Nasr who has said the above hadith

Source: Usool Al-Kafi, Volume 2, Hadith #3138

Allamah Majlisi – Sahih/Authentic (صحيح) [Mira’at Al-Uqool, Volume 12, Page 79 (See Image)]
Bahboody also said this hadith was Sahih (صحيح)