3. made lame by fortune's dearest spite: i.e., handicapped by personal injuries at the hands of fortune.

4. comfort of: delight from.  truth: personal integrity.

5. birth: i.e., high birth, social status.  wit: intellect.

7. Entitled in thy parts do crowned sit: are rightfully awarded kingship of all of your good qualities.

8. make my love engrafted to: graft my love upon.  this store: i.e., the accumulation of riches mentioned earlier: "beauty, birth, or wealth, or wit, / Or any of these all, or all, or more."

10. this shadow doth such substance give: —The "shadow" is his "engrafted" love; he feels enriched by the richness of his beloved's personal qualities. Also, Shakespeare here plays with Plato's idea that the things that we see and touch are mere shadows of essential reality.

13. Look what: whatever.

14. This wish I have: i.e., my wish that you have all the best is already fulfilled.