>>42398319
Comforting tendencies is the answer.
You have a mare who, even before you might pile on more trauma, has abandonment issues and beats up on herself, and you've got one of the more by definition understanding, compassionate ponies to be nurturing towards her, help her see light in the world again, be patient with her.
Also they both know how to tease, both can be nerds, and they're both rather likely to be ready to kill if it were necessary.
They might be good for each other in mutual grief, in one take; this isn't a pairing that is *typically* thought of in an overall happy scenario. Well that, or you can, say, go with Starlight being Celestia's student in an alternate scenario while also being a few years older than Twilight would've, and thus be in closer proximity to Cadance.
