I love that line.
This being Wagner's version of the tale, it gets weirder. We learn that when Tristan killed Isolde's fiance he was mortally wounded himself and was taken to her to be healed. She saved his life but then demanded revenge. She held his sword to his breast, but she couldn't go through with it.
Tristan is heir to the king of Cornwall, a childless old widower who does not want to remarry. After he killed her lover, Tristan convinced the king to marry Isolde. (This would almost surely lead to his disinheritance when Isolde bore a son, and would be hell for Isolde, so it was both an attempt at atonement - giving his power to her children - and an act of mutual destruction.)
As Tristan takes Isolde to England to marry the king, she is wearing around her neck the fragment of his sword that she found embedded in her lover's severed head. Her mother sent along a love potion to help ease the horror of marrying the old man. For the same purpose, Isolde brought poison.
The atmosphere on the boat is pretty hostile. Isolde demands that Tristan let her kill him. He gives her his sword and bares his breast but again she can't bring herself to do it. She takes her maid aside and tells her to fill a cup with poison, and she asks Tristan to drink a toast to peace. Tristan understands that the cup contains poison. They both drink.
But the maid switched the poison for love potion, so instead of mutual suicide they fall into a grand passion.
Almost immediately after the potion is drunk the maid wails, "You chose a swift painless death but I gave you a shameful and painful one." They're not listening. In the second act they sing love songs. In the third act they die.
I recount the plot of Wagner's Tristan und Isolde because it so perfectly captures why I love opera. It's the iconic nature of the stories that resonate in the universal unconscious, mixed with soul-melting music for an emotional double whammy. The effect is heightened when shared in the dark with a couple of thousand people.
How can a suicide pact between enemies lead to tragic love? Or turn that around: how can it not? There is a connection between hate and love that's difficult to pin down, which perhaps is why Wagner takes five hours to describe it to us.