Someone who hasn't seen or read this play might be surprised at the ending. After several extended wooing sessions, many sighs, far more love poems than we thought possible, a masque and a play-within-a-play, what do we get? No marriages. Not one.
A Hollywood producer would drop this one right in the toilet. Even Berowne, one of the main characters, is a little skeptical:
Our wooing doth not end like an old play. Jack hath not Jill. These ladies' courtesy Might well have made our sport a comedy. (5.2.947-949)
Shakespeare wrote the comedy A Midsummer Night's Dream around the same time as Love's Labour's Lost, and that play concludes with multiple marriages. In fact, the magical character Puck even states that "Jack shall have Jill" – the exact opposite of the quote above. Shakespeare has a more complicated, ambiguous resolution in mind for Love's Labour's Lost. The ladies set challenges to their men, ranging from medium-high intensity (wait a year for me) to very high intensity (wait a year for me in a monastery).
The Princess instigates this move for a number of reasons. She's shocked by her father's death and entering a mourning period when an engagement might be inappropriate. (Remember how miffed Hamlet got about the funeral meats at his mom's second wedding?) And besides, she needs to test a young man who has already proven he can't keep a vow. The other ladies naturally follow suit. They can't exactly return to France flaunting a big diamond when their king has just died.
While the play begins with a vow of celibacy that is meant to facilitate self-improvement, it ends with a vow of celibate constancy. The men are asked to focus on something outside themselves—Berowne is even asked to minister to the poor. The plot resolution is ambiguous. We believe the couples will eventually marry...but we can't be sure.
This ambiguity is heightened by the rustics' song of Winter and Spring. In it, love and youth brush up against hardship and death. "That's life," Shakespeare seems to say.