Couplet Summary

Line 13

Thus do I pine and surfeit day by day,

• The Oxford English Dictionary lists under the verb "pine," definition 4.a, "to become exhausted or wasted from physical or emotional suffering, esp. from hunger, disease, or grief." Based on the context of the poem so far (and of the rest of this line), it's pretty clear that Shakespeare has the "become wasted by hunger" meaning at the forefront of his mind here.
• The word "surfeit" also has a relationship to food—but with the opposite meaning from "pine." That's because "surfeit" means to have too much food, or to be excessively stuffed.
• So, basically, the speaker is saying that he is suffering from poor eating habits. As we know from the poem up to this point, this is a metaphor for his relationship with his beloved.

Line 14

Or gluttoning on all, or all away. 

• The grammar of this line is a little weird. Let's see if we can clear it up a bit. First of all, you should be aware that in Shakespeare's time, instead of saying "either… or," you could just say "or… or" instead. (Similarly, in modern Spanish, you say "o… o," and in modern French you say "ou… ou.")
• So, the first level of translation is to rephrase it as, "Either gluttoning on all, or all away."
• The next tricky thing is the grammar of the last little phrase. This looks weird because Shakespeare has left out some key words that you just have to mentally supply for yourself. This might not have earned him top marks in English class—but then you don't get to be the greatest poet in the English language by strictly following all the rules.
• The next level of our translation is to rephrase it as follows: "Either gluttoning on all or starving because all is away." Or, to make it even clearer: "Either gorging on everything, or starving because everything is gone." Do you see how this works?
• Now that we understand what Shakespeare is saying here, we can all breathe a sigh of relief that he found a better way to say it than we just did—even if things become a bit more unclear in the processes. 
• As you can see, the thought of this line pretty much picks up the same idea as the one before it. The speaker's relationship metaphorically veers between feast and famine, leaving him sick both ways. The final line completes the poem's processes of turning its initial idea—that "you" are to the speaker as "food to life"—into something deeply negative.
• Way to go, Shakespeare. Thanks for making us all depressed. Though we've got to hand it to you, you sure showed a lot of poetic finesse while doing it.