Love is presented as the subject or the doer of actions in the poem. Why do you think the poet has used this form rather than involving human agents?

It is not surprising that Shakespeare has chosen love as the subject and has not involved human agents. The reason for his choice is obvious and it the underlying theme of the poem, the fickle nature of humans. However, the love, and eternal being, is constant. Hence, the poet when talked of all the qualities that love embodies and should be sans of, he chose love itself and not a human, whose emotions and sentiments undergo frequent changes. Shakespeare should be appreciated for his understanding of human nature and thus a wise decision made by him in his composition.