The speaker admits to a habit that most of us share: wanting more and complaining quickly. In the first line, even a hundred sacks of grain can't fill the speaker's hunger, which makes appetite sound bottomless. Then the complaints pile up. One dry day is enough to make the speaker shout at the sky, as if the rain were owed. Play can fill a whole day and still feel dull, while a single night of learning feels like Hell.
The poem is built on these pairs, where the speaker wants a lot of what's easy and gives up fast on what takes effort. The closing line, "O, what ungrateful of a man!", turns the poem into self-criticism. The speaker isn't pointing at anyone else. The mood is rueful and a little wry, like someone catching themselves mid-grumble. The title, "Neverending Thirst," ties it together: the hunger and the thirst are more than physical, and they point to a restless dissatisfaction that gratitude never quite settles.