The speaker is talking about falling in love with one particular person, and the poem's whole trick is a repeated turn: "I didn't have to... But I did." Each step toward her, learning who she is, meeting her, seeing her, is described as something that was never required. Nothing forced the speaker's hand. Yet each time, they chose it, or at least gave in to it.
The last stanza changes the pattern slightly. "I don't have to love her / But I do" moves into the present tense, so the feeling isn't a past event but something still going on. The opening line, "It doesn't have to be her / But it is," sets this up: the speaker admits it could have been anyone, and it still turns out to be her.
The mood is quiet, plain and a little helpless. There's no flowery praise of the girl or woman herself. Instead, the short lines and steady repetition feel like someone admitting, almost with a shrug, that love doesn't follow logic or obligation.