How is the nature of love characterized in Astrophil and Stella? How successful is Atrophil's suit? How convincing is Astrophil's love?
It is possible to argue that in Astrophil and Stella, Sidney presents a highly ironic view of secular, earthly love. Astrophil seems mainly interested in Stella's physical beauty and in her alluring flesh, which he desires erotically and sexually. He seems little interested in Stella as a human being created in God's image, with a soul and a character worth appreciating in and for themselves. Partly for these reasons, his desire for Stella is ultimately frustrated. He never succeeds in loving her sexually because he never succeds in loving her spiritually. It is possible to argue that if Astrophil had loved Stella properly -- as a fellow creature worthy of true, selfless love -- he might have succeeded in winning her full love in return. (This is what happens in Edmund Spenser's sonnet sequence, Amoretti.)
A good guide to the meanings Sidney seems to intend can be found in sonnet 5, which begins "It is most true that eyes are formed to serve." In this poem, Astrophil admits his knowledge of one Christian truth after another. He admits that the senses were created by God to serve the mind or soul (1-2). He admits that the mind or soul ought to rule a human being completely (2-3). He admits that anyone who fails to follow the dictates of the reason or spirit is a rebel against Nature -- that is, against the very way he was designed by God. He admits that such rebellion is ultimately self-defeating and brings pain to the person who rebels.
Furthermore, Astrophil also admits in sonnet 5 that the sort of selfish love or desire associated with Cupid is a kind of idolatry that damages both the body (the "church") and the soul (the "churchman"). He furthermore admits that the only real kind of beauty that exists and survives is virtue or goodness. Any merely physical or material beauty is simply a shadow of true, spiritual beauty, which derives from God.
Finally, in sonnet 5 Astrophil admits that all humans are merely pilgrims on this earth, created by God in order to return spiritually to their true "country" (13), which is heaven. Yet in the final line, after admitting all these standard Christian truths, Astrophil proclaims that he "must Stella love" (14). This claim is ironic for several reasons. First, Astrophil, like all human beings, has free will, and so there is no reason that he "must" love Stella. Second, it is not really Stella whom Astrophil loves but rather himself. (He merely desires Stella's body.) Finally, the last three words are also ironic because the "love" he proclaims for Stella is not true spiritual love but is instead mere physical lust.
Little wonder, then, that in sonnet 18, Astrophil is depicted as an irrational and immature youth who has wasted God's gifts, or that he is presented in sonnet 21 as a headstrong youth who rejects the wise philosophical advice of a good, concerned friend. Nor is it surprising that in sonnet 52 he openly admits that all he truly cares about is Stella's body. Finally, it is also unsurprising that in sonnet 71 this unabashed physical lust is emphasized once again.
In Sidney's sonnet sequence, as in so much other Renaissance love poetry, true love is associated first and foremost with love of God; false love (mere lust) is the product of prideful love of self.