At first, the speaker seems to be talking about loneliness, but the poem quickly turns that idea around. They like being alone, and the reason isn't that they dislike people. It's that solitude gives them privacy for something tender: smiling at messages from someone they care about.
The feeling underneath is a quiet, slightly embarrassed happiness. The speaker doesn't want to be seen lit up by a text, which suggests the feeling is new, precious, or not something they're ready to share. Being watched would make it feel exposed. Alone, they can let their face do what it wants.
The word "smile" is set apart on its own line, so the reader almost sees it appear, small and private. The "maybe" at the start adds a shy, thinking-aloud tone, as if the speaker is only now realizing why they act this way. The mood is warm, gentle, and a little secretive, like a confession whispered to the very person who causes the smile.