Reyna waking up still in her sanctuary, looking around and seeing Sage in pain and unable to revive anyone else. Brim, Raze and Sova are all still dead.
Reyna doesn't think about it at the moment because Sage is suffering and she needs to help her and the others back to safety.
Once they are back to the VP, she keeps thinking. Sage could only revive brim a couple days later. The others took even longer. Killjoy was crying over Raze's body in a cryogenic chamber. And for a week, Sage couldn't do anything about it.
Reyna keeps going back to the moment she woke up and saw everyone else dead. Saw Sage suffering on top of her. Keeps thinking about how irresponsible it was to revive her first instead of Brim.
After Miks arrives, the team's morale improves. She sees Sage smiling more, despite the pain. Realizes how she can't help but smile whenever Sage smiles. Somehow, she becomes aware of this feeling of fulfillment she felt before but never had the time to acknowledge. It happened when they both smiled at anything.
She hears about Sage’s plan to go to China.
Alone in her room, she goes back to the moment she was revived once again. And that's when it feels wrong to let Sage go alone, fells wrong to be where she was.
Maybe Sage also felt this way when she revived Reyna. Felt like Reyna's place was by her side. Reyna felt it so strongly that she sat down to write Brim an email. The place she was needed most was by her side. And that's where she'd be.