Cleon Williams plays with a small paradox: homesickness, usually thought of as a painful feeling, is described here as something to enjoy. The speaker claims the best time to feel it is on the way home, when the ache of missing a place is about to be cured. Longing is sweeter when you know it is nearly over.
The shape of the poem helps this idea land. The first two lines are short and measured, almost like a calm statement of fact. Then the last line stretches out, bursting with an exclamation mark, as if the speaker can't hold back their happiness. The word "joy" stands out because it sits so oddly next to "sick" in the title.
The mood is warm, light and a little playful. The speaker seems to be smiling as they travel, savoring the anticipation. The poem suggests that missing something and being close to having it again can be a pleasure in itself, not just a hurt.