In these two lines, Romeo returns to his earlier theme of the contradictory nature of love. The first line is a variation of the proverb “Love is a madness.” But Romeo calls it a “discreet” madness, which, in this case means sensible or showing good judgement — the exact opposite of madness.  The second line cites two flavors that were considered proverbial opposites in Shakespeare’s time: bitter gall and sweet honey. Gall is the horrible tasting bile stored in the gallbladder of animals. Eating meat tainted by just a small amount of gall will make one choke (even die, it was said), while honey has the opposite effect. Honey is “preserving” in two ways: it’s a preservative of food, and it provides sustenance for life.