«My love is strengthen’d, though more weak in seeming;
I love not less, though less the show appear».
To justify not writing verse about the young man, the poet argues that constantly proclaiming love for someone cheapens the genuineness of the emotion.
My love is strengthen’d, though more weak in seeming;
His tone is cautious because he detects a change in his feelings for the youth: “My love is strength’ned, though more weak in seeming; / I love not less, though less the show appear.”
He recalls the formation of his relationship with the youth — rather than the current status of the friendship; this recollection is now the only inspiration for his writing and emphasizes just how far apart the two have grown. Because he expects the youth to be indifferent, he is firm but courteous when he refrains from writing verse: “I sometime hold my tongue, / Because I would not dull you with my song.”
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com