Why is Eros a tyrant in Edwin Arlington Robinson's Eros Turannos?

Eros was the god of love, and Aphrodite's son; Latinate: 'sexual love.' Eros is not the tyrant, the title means: "love the tyrant." Presumably, then, the male subject is the tyrant, and the poem is about her melodramatic affections for him. You could, though, take the title to say: "Love, the tyrant," in which case you are quite right to pose this question in this way. The quick answer might be that she [the subject] is quite in the grips of love. Despite the almost demonizing descriptions of the male, though, she cannot seem to rid herself of him. She has "all reasons to refuse him," and yet she does not. This would be the superficial answer, and obviously more care might lead you to construct a much better reading of this, but I hope that this is something of a start.