My hunch is that it's not the good cheer of such folks that become difficult to take, but rather the sheer volume of nervous energy they put out. Hyper, like those little yappy lap dogs.
Quite frankly, I should know, being married to a lady with a bit of that temperament. Not every day these days; she's too ill for that. But on her "better days", yes.
In Pam's case, she's severely mentally ill (and owns it--I'm not telling tales out of school here) but also "high functioning".
When she gets really quiet, she's usually either in ultra-severe pain or severely depressed.