The speaker starts with a simple question: when was the last time they were truly happy? The trouble is, they can't answer it. Nothing comes to mind, and that blank leaves them unsure how to feel. "Is that sad? / Is it bad?" sounds like someone checking whether their own reaction is normal, almost asking the reader for permission to feel the way they do.
The middle of the poem draws a quiet line between two states. "Content" is what the speaker settles on, and the phrase "I guess" makes it sound lukewarm, more like a shrug than a comfort. Then comes the flat, repeated "Never happy," which hits harder because it's so short. The final question, "What is happy?", suggests the speaker may not even be sure what they're looking for. The short lines and bare wording give the poem a hesitant, tired, slightly lost mood, as though the speaker is thinking aloud and hasn't reached an answer.