What does the tear in Sir Patrick's eye reveal in "Sir Patrick Spens"?

The tear reveals that Sir Patrick is not happy or optimistic about the mission he's been asked to undertake.
When Sir Patrick Spens first reads the letter from the King, he laughs. The notion of sailing to Norway in a dangerous season for such a voyage is ridiculous to him. After he has a moment to process the request, his eyes are blinded with tears. He's sure that the mission won't be good for him; he doesn't believe he'll make it home safely.
He wonders who suggested him for the trip and wishes they hadn't done so. Still, he decides that he has to do as the King of Scotland requests. He's going to sail to Norway and retrieve the daughter of the Norwegian King. The winter sea may be dangerous, but he will not refuse the royal command.