It is easier to relate with something you know of or have felt before. This works in both long term and short term. If you have seen or felt death or pain, you can relate to someone feeling it at the moment. If you're currently in pain, you can see the pain in other people more likely.

Perhaps this is where the saying "it takes one to know one" comes from. If you are a person that is often paranoid, wouldn't you see others that are also paranoid? Wouldn't you become observant of the tiniest details or actions of other people and declare it as a symptom of paranoia?