do you know anyone that has a mental disorder?
I know a number of people my current job is case management and we only deal with clients with disabilities from metal illness, behavioral and physical impairments. 
Oh wow i bet that is tough work.  My dad has dementia which I guess would be considered a mental disorder
Yes it is at times it can be very stressful and tough, and does your dad have symptoms of emotional problems or decrease in movement due to his impairment. 
yes, he definetly has the movements and anger issues.  so it is rough at times
Has he been evaluated for bipolar disorders that may be the reason his moods is so bad because his moods are often elevated.
does that go along with dementia?
Yes that can be a factor they are close because the moods change from happy, to sad and even irritable a lot at times.