Sore legs trigger two very different memories for the speaker. At first, the ache feels familiar and even warm, recalling childhood games of tag, when running with friends was pure fun and the tiredness afterward meant a good time. The title, "Tag, you're it," points to that playful world, where being chased was a game rather than a threat.
Then the poem turns. The same pain now reminds the speaker of running from their problems, which is a much heavier kind of exhaustion. The repetition of "This pain in my legs" makes the turn feel sharp: the sensation hasn't changed, but its meaning has. The speaker wishes the pain came from playing with friends instead.
The ending is short and plain: "I wish things were simpler. / I want to be a kid again." The mood is wistful and tired, with a quiet sadness. The simple language suits a speaker who longs for an easier time, and the poem's honesty makes that longing easy to relate to.