Sonnet 75 centers on some logic that is the bane of anyone's existence who has ever studied for their SATs. What could this little hobgoblin be, you ask? Why, analogies, of course! You know, like: "A is to B as C is to D." Usually, this sort of puzzle is presented with three terms filled in, and one left blank.
The poem's speaker hits us up with one of these crazy analogies in the very first line, telling us that "you" (A) are to "my thoughts" (B) as "food" (C) is to "life" (D). In Line 2, the poem fleshes out this initial idea by saying that "you" (A) are also to his "thoughts" (B) as "sweet seasoned show'rs" (pretty, pretty rain) (C) are to "the ground" (D).
But then at the end of Line 4, Shakespeare throws a stick through the spokes: now he says that the analogy between "you" and the speaker's "thoughts" is also the same as that between "a miser" and his "wealth."
With this third analogy, the speaker seems to have finally hit on something, because the majority of the poem from now on (from line 5 until line 12, that is) explores the similarities between his feelings of love and the life of a miser. The picture that emerges in these lines is much more tormented than the opening lines of the poem might suggest.
It seems that the speaker can't make up his mind at all: does he want "you" all for himself, or does he want to show "you" off to the world? Is he sick of "you," or can he simply not get enough of "you"? The speaker has no idea.
The poem ends by describing the speaker's life as swinging wildly between two excesses—and here, the food metaphor comes back in. Either the speaker is starving for "you" or he has gorged himself so much on "your" presence that he is sick of you. Nice!