What is the theme of "Sonnet 91"?
This sonnet, like many others by Shakespeare, concerns itself with how we define ourselves and what gives us glory and greatness in the eyes of the world. The poem begins by listing examples of some of the things that provide men with status: birth, skill, wealth and possessions are all mentioned as bestowing upon some men glory. However, to the speaker of this poem, these "measures" of glory are not relevant to him. The speaker has only one measure that determines whether he has importance and status or not: the love of his beloved who is addressed in this poem:
Thy love is better than high birth to me,
Richer than wealth, prouder than garments' cost,
Of more delight than hawks or horses be;
And having thee, of all men's pride I boast:
Thus the love of the beloved makes the speaker feel as if he is envied by all men, no matter what their birth or how wealthy or what possessions they have. The love of the beloved allows the spaker to boast "of all men's pride." Yet, the final couplet also identifies that this is both an exhilirating and a very vulnerable state, for, if withdrawn, the absence of this love could reduce him to nothing:
Wretched in this alone, that thou mayst take
All this away and me most wretched make.
Building your own sense of importance and status around the love of another always leaves you vulnerable to being made "wretched" if that love is ended for whatever reason.
In this sonnet, Shakespeare lists many of the things that men (and note that most of the items he lists are specific to male measures of success) prize as "measures" of a happy life. Some prize aristocratic lineage ("glory in their birth"), some their innate gifts ("skill"), some riches, some their strength ("body's force"), some their ability to dress well, and some their ownership of hunting hawks, hounds, and horses. He manages to get a jab in at overdressed, foppish courtiers ("new-fangled ill").
But for Shakespeare, the regard of his beloved is far more important than any of the things listed. However, as he states in the final couplet, if the loved one chooses to withdraw this most-prized affection, he will be left wretched and bereft of what is his true "measure" of happiness.