Imaam Ibn al-Qayyim رحمه الله said,
"Allaah سبحانه و تعالى has commanded (with gratitude), prohibited against its opposite, praised its people and described them as the most special of His creation. He made (gratitude) to be the objective behind His creation and command, promised its people with the best of His reward, made it a cause for increase in His bounty and as a guard over His blessings. He informed that its people are the ones who benefit from His signs. The (linguistic) foundation of shukr is the appearance of the effect of nutrition upon the bodies of animals in a clear way. It is said shakirat ad-daabbaa, tashkaru shakran, when the effect of consuming fodder becomes visible upon it.
The reality [of gratitude] in relation to one's servitude [to Allaah سبحانه و تعالى] is the appearance of the effect of the favour of Allaah سبحانه و تعالى on the tongue of His servant in both acknowledging [the favour] and praising Him for it. Likewise, upon his heart in terms of witnessing [the favour] and loving [Allaah سبحانه و تعالى for it]. And then upon his limbs, in outward compliance and obedience.
Thus, gratitude is built upon five foundations:
a) Humility towards the One being thanked,
b) Love of Him,
c) Acknowledging His favour,
d) Praising Him for it, and
e) Not utilising it in what is disliked by Him.
So these five comprise the foundation of gratitude and it is contructed upon them. When any one of them is absent, then a foundation amongst its foundations becomes defective. The speech of everyone who spoke about gratitude and defined it returns back to this and revolves around it. It is said, "Its defition is acknowledging the favour of the bestower with humility," and it is said, "Praising the benevolent one (who bestowed the favour) by mentioning his benevolence," and it is said, "The heart settling upon love of the bestower, the limbs upon His obedience and the tongue flowing with His mention and praise," and it is said, "It is to witness the favour and to preserve the sanctity (of its prohibitions)."
The gratefulness of the common-folk is for food, drink, clothing and bodily nourishments. The gratefulness of the special ones is for Tawheed, faith (Eeemaan) and the nourishment of the hearts. And gratefulness is always accompanied by increase, due to His saying, the Exalted:
"And [remember] when your Lord proclaimed, 'If you are grateful, I will surely increase you [in favour]; but if you deny, indeed, My punishment is severe.'" (Soorah Ibraaheem, 14:7)
So when you do not see your condition in (a state of) increase, then turn to gratitude."
[Refer to al-Madaarijus-Saalikeen, (2/232 onwards), abridged. As quoted in the appendix to the English translation of Qaa'idah fis-Sabr wash-Shukr (A Principle Concerning Patience & Gratitude) by Shakhul-Islaam Ibn Taymiyyah رحمه الله, Hikmah Publications]