Like Cather, I researched this. My favorite site for Greek words is blueletterbible.org [Use the KJV + Strongs, enter the verse or word and search on]. Of course I got the same result

nah-SAY-oh G3552--used only once in the bible--actually means "sick" and metaphorically 'mentally ill'. So either of the NWT's definitions are appropriate.

As for the reason to changing, I think it's about one expression being more 'politically correct' than the other. In our culture calling someone "mentally deranged" will cause offence. "Obsessing" on the other hand, is unlikely to elicit the same reaction because most folks can admit to obsessing at one time or another. IMHO this change was a pretty smart move.

Grandma Cee Cee