"Their quietness ought really to be seen as an expression, not of coldness, but of their sincerity and seriousness of purpose."
ISFJ type description by J. Butt and M.M. Heiss :
"One ISFJ trait that is easily misunderstood by those who haven't known them long is that they are often unable to either hide or articulate any distress they may be feeling. For instance, an ISFJ child may be reproved for "sulking," the actual cause of which is a combination of physical illness plus misguided "good manners." An adult ISFJ may drive a (later ashamed) friend or SO into a fit of temper over the ISFJ's unexplained moodiness, only afterwards to explain about a death in the family they "didn't want to burden anyone with." Those close to ISFJs should learn to watch for the warning signs in these situations and take the initiative themselves to uncover the problem."