I am not a biologist nor know much about biology (so please explain in layman's terms) however I have always been curious as to why this is.
What causes the difference in pain between touching an open wound and pushing down on your skin? In both circumstances you are applying pressure to the nerves except one is a foreign object touching the nerve and the other is the nerve's natural surroundings in your body.
Why is it that a foreign object causes so much more pain?
I answered the other question and it is similar. However, this one is different enough that I will give a slightly more simple explanation.
Nerves transmit messages using electrical current, more specifically the flow of sodium and potassium. Just remember that it's electricity. When you have any sort of cut or damage, your body recruits white blood cells which protect you from any bacteria that might enter. They also cause inflammation to prevent those bacteria from spreading (and to warn other cells that they may become infected). It is also important that we feel pain, so we know something is wrong (because we have the ability to do something about it). That's why inflammation causes pain. It does this by lowering our threshold so our nerves are more sensitive. In other words, it takes less damage, pressure etc. to cause our nerves to send the pain signal. Normally, it is set higher, so we don't suffer too much pain. Naturally if you want clarification on anything just ask.
