Tone is the attitude, feeling or emotion a narrator (prose) or a poetic persona has for the subject being discussed or described. Some examples of the many possible tones are derision, admiration, horror, or joyfulness. Tone is distinct from and different from mood, which is the emotion that the setting, diction, description and imagery inspire in the reader.
Petrarch's Sonnet 90 is a bit tricky to understand unless you pay careful attention to the language and follow carefully the twists he makes in his discussion of the subject of someone he once loved. (Did he only once love her?) Let's journey through the sonnet and find what he is saying so we can find the tone he says it with.
Petrarsh's poetic persona (agreed to be Petrarch himself) tells of the fair maid he once loved who let her hair "fly free." This implies that once she was young but is not now. Thus if she is not young now, neither is he young now. He says pity looked at him from "those deep eyes" suggesting the unrequited (unreturned) love that Petrarch's sonnet cycle tells about.
He says love burned brightly "heaped within my breast" and asks that, considering her eyes shone so bright and pityingly, could anyone wonder that he was so deeply in love:
I had love's tinder heaped within my breast;
What wonder that the flame burnt furiously?
He praises her beauty and angelic qualities, then admits that others will say that her angelic qualities are dampened and don't shine out in her old age: "You say she is not so today?"
Petrarch ends with the paradoxical twist to his sonnet resolution that says through two implied metaphors that even though Cupid's bow is "unbent" (at rest, no arrow of love aimed at him), his wound of Love from Cupid's earlier arrows causes him to still love her and adore her, though without youth's passion.
The tone of the admiring poetic persona is gentleness, nostalgic, and kindly: he is narrating a fond emotion and an even fonder reminiscence of by-gone days. He has a tender tone even though he never won her love and the pity toward him for his unrequited love is gone from her eyes.
She seemed divine among the dreary folk
Of earth. You say she is not so today?
Well, though the bow's unbent, the wound bleeds on.