There's a question that sounds simple but isn't: what's the point of saying thank you to someone who doesn't need anything from you?
That's the question behind gratitude in Islam. Allah isn't more complete if you thank Him, nor poorer if you forget to. And yet the Qur'an promises something concrete to those who are grateful: "لَئِن شَكَرْتُمْ لَأَزِيدَنَّكُمْ" ("La'in shakartum la'azīdannakum"), which translates as "If you are grateful, I will surely increase you."
The obvious answer would be: gratitude is for us, not for Him. But there's a story that shows this more concretely than any theoretical explanation. Aisha (ra) once asked the Prophet ﷺ why he kept praying so much at night, to the point his feet would swell, when Allah had already forgiven all his sins, past and future. He answered with a question: "أَفَلَا أَكُونُ عَبْدًا شَكُورًا" ("Afalā akūnu ‘abdan shakūran?"), "Should I not be a grateful servant?" (Hadith, Sahih al-Bukhari and Sahih Muslim).
That answer doesn't do what you'd expect. It doesn't say "because I fear punishment" or "because I still have sins to be forgiven." The person with the least possible reason to worry about Allah's forgiveness, since it had already been guaranteed, chose to stay grateful anyway.
That changes the question. It's not "what do I still need to ask for," it's "what have I already received and never stopped to acknowledge." Most of us spend more time on the first question than the second.
There's a simple line that sums this up: the ungrateful see what's missing, the grateful see what they already have. It's not forced optimism, it's just a different starting point. The same life, looked at from two different places.
What I find hardest, in practice, isn't feeling grateful during good moments. It's remembering it precisely when things aren't going well, which is exactly when it stops being easy and starts meaning something.
The promise in the Qur'an, at its core, is about seeing more of what you already have, not about having more things.