Two lines are all this poem needs. It begins with a question, "What is your saddest story?", and the answer that follows is strikingly small. There is no death, no betrayal, no dramatic event. The saddest thing the speaker can think of is a failure to say something: "I never told him how much I love him."
That plain answer is what gives the poem its weight. The speaker doesn't dress up the regret or explain it. The grammar is even a little awkward, as if the words came out raw and unpolished, which makes the confession feel honest. The tense is worth noticing too. "Love" is present, not past, so the feeling hasn't faded. We can't tell from the text why the words went unsaid, or whether he can still hear them now, and that open gap is part of the ache.
The mood is quiet, wistful and tender. It leaves the reader wondering who in their own life is still waiting to be told.