The speaker of Sonnet 75 is desperately in love, but he talks about it as a kind of wealth. He compares his relationship with love to that of a miser to his money. And this poor miser is never satisfied. Because he is so terrified by the idea of spending his riches, the miser can't enjoy what he has. But, as soon as he starts thinking about showing off his wealth to the world (you know, with a grill or something), he starts worrying that other people will steal it. Thus, Shakespeare's imagery of greed shows how a person can become trapped in a vicious circle of his or her own making. This isn't to say that the poem is condemning money or wealth—you'd think it would be possible to find a happy medium. But Shakespeare doesn't show us any happy medium in the world of this poem. The miser's relationship to wealth shows us a person who can only experience extremes.
The miser imagery is similar to the glutton imagery, because neither of them can be satisfied with what he has. The miser is different from the glutton, however, because the glutton isn't afraid that other people will steal what he has.
Shakespeare puts the miser imagery in the middle of the poem because its negative imagery makes a good transition between the positive nutrition imagery of lines 1-2 and the negative nutrition imagery of lines 9-14—and not, you know, because "miser" and "middle" both start with M.