Suffering is probably The Idiot's one universal constant. However wealthy, healthy, or set up, each character suffers a lot mentally or even physically. Often this happens at the hands of those closest to him or her. The novel's suffering runs the gamut: there is illness, depression, suicidal tendencies, romantic betrayals and upheavals, alcoholism, poverty, shame, and of course, for the piece de resistance, murder. Witnessing suffering tends to be a way to test characters: in the face of suffering most are cynical or dismissive, trying as much as possible to keep themselves from having to deal with the pain of another.
The novel's world is so filled with externally inflicted suffering that the best strategy to cope is to adopt Myshkin's water-off-a-duck's-back approach to just universally forgive everyone for everything instead of being constantly offended by those who wish to harm.
Suffering is a high Christian value, and Myshkin, as the Christ figure, places a lot of store in his ability to read people's faces for signs of suffering. However, the novel makes it clear that most suffering is in fact meaningless and ultimately neither helps the person enduring it, nor those watching.