The cutting portion of the needle will become extremely dull after being used once compared to being unused. A dull needle will cause more unwanted trauma and discomfort for the piercee as opposed to a new unused needle.
Cody Vaughn - APP Outreach Committee
**MORE FROM RYAN BELOW**
Piercing needles are very finely sharpened, they are not designed to be used for more than one puncture. Here are some pictures that illustrate how quickly a beveled needle dulls from use. These are pictures of syringe needles (very similar in design to piercing needles) under magnification after use.
So thats why the 2nd nipple hurt more than the first.