Instead of listing the usual reasons for loving someone, the speaker gives one small, honest answer: around this person, their smile stops being fake. The poem opens with a question, "Why do I love you?", and the "Maybe" that follows makes the answer sound like something the speaker is still working out, not a firm conclusion.
What stands out is the quiet admission underneath. If the speaker only now realizes the smile "isn't fake," then smiling has probably felt like a performance in other places. Judging by the wording, love here means being relieved of that pressure and feeling safe enough to be genuine.
The mood is tender, gentle and a little vulnerable. The short lines and the abrupt ending on "Isn't fake" make the poem feel like a thought spoken aloud, simple and sincere, with no need for grand declarations.