Didn't they say in Enterprise that the Klingon augment virus caused personality changes?
Laneth (a infected female Klingon warrior) claimed that she "felt fear for the first time since I was a child."
Fictional characters, just like real people, say all sorts of things that aren't necessarily objective fact. Klingons believe that humans are weaklings, so it's natural that Klingons who've been altered into a more humanlike form would expect to be weaker as a result, and that this could become a self-fulfilling prophecy, altering how they perceive their own behavior and feelings. Self-image has a lot to do with how we think and behave. So it's hardly evidence of a genuine personality alteration.