"empathy" and/vs "sympathy"

Not to get into a pissing contest about it, but I woke up this morning thinking that "empathy" is one of those tales people tell themselves that subtly elevate their standing and simultaneously fall short of accuracy. This is not to say there isn't a potentially-benevolent social fallout, since taking others into consideration really does make life a little less isolated or grim.

But isn't there something to be said for the fact that there is no way, in actual-factual terms, for anyone to know with perfection what anyone else is suffering or enjoying?

Here is one Internet approach to the topic, for example: "Empathy is the ability to experience the feelings of another person. It goes beyond sympathy, which is caring and understanding for the suffering of others. Both words are used similarly and often interchangeably (incorrectly so) but differ subtly in their emotional meaning."

In order to experience what anyone else might experience, wouldn't it be necessary not just to be currently experiencing something but also to have lived a lifetime precisely as the other person had? And isn't that impossible?

This is the kind of discussion that is likely to piss off the tree-huggers, but that's not my intention. My curiosity centers on the degree to which self-deception is a requirement or demand.

It also raises the question in my mind: If I knew you or you knew me perfectly, would that be a blessing or a curse? It sounds horrific to me, but I could be wrong.