According to the Online Etymology Dictionary, the word "sad" comes from the Old English sæd "sated," from base *sa- "satisfied." This led to an aha moment. How can "sad" be related to being "satisfied"? Like this:
Sense development seems to have passed through a meaning "heavy," and "weary, tired of" before emerging c.1300 as "unhappy."(The entry also gives an "alternate course" for the word through "steadfast, firm," and "serious" to "grave," but doesn't cite the words that occurred through this development, and anyway, the first was more interesting to me.)
So what is contentment, then? Temporary, at least, or so I've noticed.