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an exhilirating and a very vulnerable state, for, if withdrawn, the absence of this love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:

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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.