Now that Lover's Oath has suddenly gone from being played only once in game to being played three times in game, I'm compiling themes relating to all three uses to see if it holds up if/when it is used again.

So it first appears when Xiao is introduced, then years later in Durin's Story Quest in his farewell scene with "Anya", and recently in Varka's Story quest in his welcome back scene:

1. Near "death" situations:
1. Xiao character story 4.
2. Durin would have returned to Simulanka and lost his life/place in Teyvat.
3. Varka was going to be merged with the leylines.
2. Defying fate:
1. Xiao's situation is unclear but considering how nearly all the yakshas died except him and how he thinks that death is a yaksha's fate, he probably defied fate by surviving.
2. Durin defied fate by writing himself into Teyvat.
3. Varka went against the fate he saw.
3. Longing:
1. Xiao's namecard references longing to put down his weapon.
2. Durin was longing for his mother.
3. For Varka, right before that scene, everyone's help/memory was labelled "XXX's longing", and it is their longing that brings Varka back in a sense.
4. Traveler and Venti's presence:
1. Traveler makes sense because generally a descender's involvement is needed to defy fate. However, Venti was also involved in both Mondstadters' story quests and Xiao's near-death situation. Since Venti's own fate is apparently unknown, it is interesting.

Note also that despite first playing in Liyue, it has a western instrumentation, and since then has been used twice for fully Mondstadt stuff. However, the Chinese name for the song basically describes the exact type of foliage at Dihua Marsh around Wangshu Inn.