By what outward signs do we truly know that somebody is in love? This is the question that this sonnet by Sir Philip Sidney seeks to address. The sonnet opens by listing the number of things that somebody who was thought to be in love should traditionally display, and which the poet does not: he does not, for example, "nourish special locks of vowèd hair," nor "give each speech a full point of a groan." The speaker here is clearly parodying the ideas and attitudes of his time as to what a lover must do to show himself to be in love.
The sonnet thus acts as a challenge of stereotypical and rather ludicrous notions of what love is and how it is to be evidenced. Love is not something that is so easily and whimsically indicated in such stereotypical actions, the poet argues. The final two lines of the poem states the speaker's beliefs about love and how it should be evidenced:
Dumb swans, not chattering pies, do lovers prove;
They love indeed who quake to say they love.
Being "dumb" is therefore better evidence of somebody who is stuck by Cupid's arrow, rather than somebody who is a "chattering pie," which therefore indicates the frivolous nature of their affection. Love is such a momentous, powerful force that it is only those who "quake" to admit that they are in the power of such a compelling emotion who are actually in love; the other signs of supposed "love" indicate nothing more than superficial emotion.
This sonnet begins with the poet describing all the ways he does not make his love obvious to the rest of the world. He doesn't talk about his love to just anyone he encounters. When he says that he is not seen "colours for to wear," he is saying that he doesn't look like a lover (which means he probably doesn't wear any tokens given by his lover), and he doesn't wear in any obvious place locks of hair that his lover might have given him. Even his speech is free of the plaintive groans that characterize the speech of love-lorn men pining for their lovers.
The rest of the Court, especially the court's eligible ladies, are convinced because of the poet's outward failure to display any of the conventional attributes of the courtly lover that he's a lost cause and resolve simply to leave him alone, convinced that "He cannot love."
The last six lines of the poem establish the poet's real condition--he is indeed in love and, most important, his true love, Stella, understands his love even though he doesn't display it. True love, according to the poet, is not worn on the sleeve "but in the heart." He uses swans to convey the truth of his love: they are silent, but, as everyone would recognize, swans mate for life.
The theme of this sonnet is that true love is not loud; it's not worn on the sleeve and displayed for all to see. True love is unspoken to everyone except the lovers, and it is forever.