Maybe, at some other time, Buffy might have taken Xander's disappearance marginally better. Emphasis on the marginally. However, considering how vulnerable she was feeling at the moment because of everything going on, it only seemed to compound her feelings of despair. She didn't usually cry like this- she felt almost ashamed that Kirk had to see it-- weak.

"Jim, I-" She began, struggling to keep her voice from shaking and to still the sobs that kept rising up from some seemingly infinite source. "-Xander's gone. I don't... I don't know where."