What's he saying?
"My love is strengthened, though more weak in seeming; / I love not less, though less the show appear;"
Even though it seems like I love you less, because I show it less, I actually love you more;
"That love is merchandized, whose rich esteeming, / The owner's tongue doth publish every where."
When someone announces the value of love all the time, it reduces that love to the level of an object that can be traded.
"Our love was new, and then but in the spring, / When I was wont to greet it with my lays;"
When our love was new, I liked to celebrate it with songs and poetry;
"As Philomel in summer's front doth sing, / And stops his pipe in growth of riper days:"
Like the nightingale sings most in early summer, and less as summer becomes autumn:
"Not that the summer is less pleasant now / Than when her mournful hymns did hush the night,"
She doesn't sing less because the summer has become less beautiful,
"But that wild music burthens every bough, / And sweets grown common lose their dear delight."
But rather because many other birds have begun to sing, making her song less precious in comparison.
"Therefore like her, I sometime hold my tongue: / Because I would not dull you with my song."
So like the nightingale, sometimes I choose not to praise you, because I prefer not to bore you.
Why is he saying it?
Sonnet 102 continues the theme of silence that was begun in Sonnet 100, and attempts to explain and excuse the fact that the poet is not inspired as of late to write poems of praise about the fair lord. In Sonnet 100, the poet blames the lack of poems he produces on the absence of his Muse, asking: "Where art thou Muse that thou forget'st so long, / To speak of that which gives thee all thy might?" Finally, in Sonnet 103, he begs, "O! blame me not, if I no more can write!" and decides that it is not so tragic if he cannot be inspired to write, since the fair lord will find beauty enough simply by looking in the mirror.
The honesty of the sonnet's message is called into question in the very first two lines, with the use of the words "seeming" and "show." These two words usually indicate falseness; if "love" is taken to mean the poet's affection for the fair lord, then the diction is in accordance with the convention of sonneteering, that the beloved is faultless. However, "love" can also be interpreted as meaning the fair lord himself, the beloved. This group of silence-themed sonnets follows on the heels of sonnets focused on abandonment (Sonnets 87-9), hatred (Sonnet 90), deception (Sonnets 94-6), and separation (Sonnets 94-6). In that light, it is possible to read Sonnet 102 as a kind of "show" itself, covering for a tired, failing love.
Philomel of line 7 refers to the classical term for a nightingale. In Ovid's Metamorphoses, Philomela is raped by her brother-in-law, Tereus. She is turned into a nightingale after she takes revenge for the rape; her sister Procne is turned into a swallow, and Tereus himself becomes a hoopoe. However, here Shakespeare does not seem to take into consideration this disturbing context; neither does he in A Midsummernight's Dream, when the chorus sings, "Philomel with melody, sing in our sweet lullaby" (II.ii.13-14).
Lines 11-14 lend an air of bitterness to the otherwise pleasant imagery of the nightingale singing. The metaphor of the nightingale falling silent in the fall is likened to the poet's choosing not to write about the fair lord as much anymore, since now rival poets are creating that "wild music" that "burthens every bough," or weighs down the branches of the figurative trees where the nightingale used to sing. The word "wild" was used as a synonym for "vile," and "burthen," which means "burden," also carried the meaning of "chorus," or "refrain." These words used together call to mind the image of a tree branch crowded and weighed down by "common" birds singing horrible songs: the rival poets.
The final couplet clarifies the metaphor of the nightingale, and the poet offers an explanation for why he chooses not to praise the fair lord among other, rival poets. The phrase "dull you" could mean "bore you," since the poet's "song" would just add to the others, not standing out as unique since they are all praising the fair lord. It also means that the poet does not want to risk heaping too much praise on the fair lord, because his praise in addition to that of the other poets might have the effect of making all praise seem "dull," or "common."
