[Asriel pulls his hand back slightly. He doesn't know what comforting thing he could possibly say to having all your friends leave. Mettaton just lost a friend before, didn't he?]

... It's not a "mistake". These were people you cared about, right?

...

Mettaton, do you think... would crying help?

[He did it before, right? When they came back from the Outpost the first time. Asriel doesn't know if he'll want to, but he'll be someone to cry on if that's what Mettaton needs.]