How would you describe the speaker’s feelings about his lover?

I would describe the speaker as incredibly devoted and absolutely smitten. He not only believes that his lover is more beautiful than a summer day, but he also has worked hard to immortalize her beauty in his poem. He wants to give her the only kind of eternal life that one person can give another, and so he immortalizes her loveliness, comparing it to an "eternal summer" that will never fade or die, unlike the actual season of summer (which passes on to fall and winter in its time). He finds fault with a real summer day, a day from, perhaps, the best loved season of them all, when he contrasts it with her. This makes it seem as though his feelings for her run quite deep and consume him; however, there is an implied endurance of his feelings. This is not a fly-by-night lust or infatuation.

There isn't one single best answer to this question. Different readers might have slightly different interpretations as to what exactly best describes the speaker's feelings about his lover. I think "deep" and "passionate" work well, because the poem is full of beautiful language that is meant to highlight how much better his love is than a summer day. Summer days are awesome, and she is even more awesome. Additionally, I would describe his love as long-lasting and committed. The final lines of the poem state that the narrator believes that his lover's beauty will not fade away. She's not an unchanging immortal, but to him, she will always be beautiful. He has no eye for any other woman. She will always be perfect to him. She might think her beauty is fading away with age, but he simply doesn't believe that her beauty will fade. It might change, but it won't fade in his eyes. Even after death, she will still be beautiful because she will be living on through the poem. I would definitely say that is some deeply, passionate, and confident love.