imagine
this is blooky’s response to things they find distressing
considering they find it hard to truly accept praise, compliments, or encouragement,

see exhibit A, it is likely these instances are a source of anxiety for them until they can see the person is really being genuine

whenever ghost!Mettaton encouraged, complimented, or praised them, their initial response might’ve been a disbelieving ‘…….oh no………’

and so we have the humble origins of Mettaton’s ‘oHH yeSSSSSSSS’