The poem is built like a short argument, and its last line is the one that carries the weight. The speaker starts with two ordinary ideas: feeling sad doesn't guarantee tears, and feeling happy doesn't guarantee a smile. Most people would agree with both. Emotions and their outward signs don't always match. Then the poem turns, using "because" to link those everyday examples to something far heavier: having a gun doesn't mean the speaker will shoot.
That jump is what makes the poem unsettling. By placing a gun next to crying and smiling, the speaker treats it almost as one more thing that can sit inside a person without showing. The poem doesn't say why the speaker has a gun or what they feel. Judging by the tone, the speaker may be trying to reassure someone, or perhaps to defend themselves against suspicion.
The mood is quiet, restrained and tense. The flat, calm phrasing makes the final line feel both comforting and uneasy, because we're left wondering how much to trust it.