Dante packs a lot into this Canto. We are going to have to attack this rather like I attack a buffet: go for what is familiar first, and then fill up the edges with the other stuff. Virgil is attempting to explain to Dante the nature of love. Think love with a capital L, and not in the Hollywood sense. This is love in its biggest sense, as the force which keeps the universe going. This is love as an inward movement of the soul. Love is something like gravity - it is the thing which (as the 60's song says) "makes the world go 'round".
So here we are, beings who are created to love. We begin by desiring things which please us. This is the first stirring of love. Since we are created in order to love, how can we then be blamed for loving wrongly? And how is loving "wrongly" even possible? As Dante puts it to Virgil: "how can you praise or blame it for its love of good or bad?" In other words, isn't there a kind of determinism about the whole thing? How can there be ethical judgment about love? Virgil begins his reply:
I can explain to you
as much as reason sees; for the rest, wait
for Beatrice - it is the work of faith.
Purgatory XVIII. 46
Here again Virgil recognizes the limits of human reason in understanding these things. For love is not a human construct; it is divine. it is both an attribute of and a gift from, God. Even, we might say with the author of John's letter, it is the nature of God himself. So neither you nor I will understand the true nature of love without Beatrice - without the divine aid of faith.
While reason has its limits, it also has its role in directing our natural inclinations, our natural "loves". We need not consent to every inclination which arises within us. And it is the faculty of reason which is charged with this task, transforming natural inclinations into 'rational' love.
you have the innate faculty of reason,
which should defend the threshold of consent...
Let us assume that every love that burns
in you arises through necessity:
you still have the power to restrain such love.
Purgatory XVIII. 63 ff
Here I think we need to turn to another poet - John Donne - who articulates something of the same sentiment in Holy Sonnet XIV. Look it up and read it through a few times. Seriously.
Reason, your viceroy in me, me should defend,
But is captived, and proves weak or untrue.
Here Virgil tells Dante that reason, exercising its ability to restrain and direct our desires, finds its expression as the "noble power Beatrice knows as Freedom of the Will" (73). And in fact the whole of purgatory is about reclaiming the freedom of will which was lost through sin.