"You just don't understand!"

Every so often you see someone compile a list of things not to tell those who is suffering or grieving. At or near the top of the list is "I know just how you feel".

The problem, though, is that many suffering or grieving people have a schizophrenic attitude toward sympathy. On the one hand, they want people to sympathize with their ordeal. On the other hand, if you express sympathy, they may be offended. "How dare you–you haven't gone through what I'm going through!" 

By that logic, it's safer to say nothing. Do they really not want people to care about their situation? A problem here is a failure to differentiate two kinds of sympathy. Consider these two statements:

i) I understand what you're feeling

ii) I feel what you're feeling

The first statement is an expression of intellectual sympathy or compassion. At a conceptual level, they grasp the situation. 

That's a necessary condition of compassion. Say you see news footage of a natural disaster than leaves survivors in terrible distress. Now the viewer, didn't experience the natural disaster, but they have the intellectual ability to project themselves into the situation: "What would it be like if I went through that ordeal?"

If we didn't have the ability to imagine what someone else is going through, there'd be little basis for compassion in general. Compassion would be limited to those having the same personal experience. "If it doesn't happen to me, it isn't real. It doesn't register." That's the mindset of the sociopath. 

It's a great mistake to dismiss intellectual sympathy. Social life depends in large part on the ability to relate to someone else's situation even if you don't share their experience. Moreover, a degree of emotional detachment can be essential to assess a situation and offer a solution (if any). 

At the risk of sounding paradoxical, consider the dispassionate compassion of a good physician. Although he cares about his patience, his medical judgment isn't clouded by emotion. 

The second statement is an expression of emotional or existential sympathy. They've had the same kind of experience or a similar kind of experience. So they can relate to the suffering individual on the same emotional level. That kind of sympathy or empathy is especially valuable in consoling grief-stricken individuals. 

Both kinds of sympathy are necessary and valuable. One is not a substitute for another.