In just three short lines, the speaker describes someone whose soul is already broken, yet who still smiles. The twist comes in the second and third lines. We expect the smile to be a mask, the usual way people cover up hurt. The speaker says no: the smile is not there "to hide the pain" but "to enjoy the pain."
That is a strange and unsettling idea. It might mean the speaker has found a kind of acceptance, where pain has become part of who they are and even holds some meaning or comfort. It could also point to something darker, a person so used to suffering that they have learned to take pleasure in it. The poem doesn't settle this, and that ambiguity is what makes it linger.
The mood is quiet, bittersweet, and a little eerie. The brevity helps. With so few words, the smile feels deliberate and the pain feels close, almost like something the speaker is holding on to.