I used to think that there was something about the lighter itself that caught Mike's attention. However, the novel specifically states that Mike was surprised that his hands were steady.

This is a scene where FFC really deserves some credit. This was a major turning point for Mike. The only line from the book is this,

"To his surprise his own hands were steady".

FFC really managed to demonstrate this as THE turning point for Mike, a point at which he trasforms before our very eyes. I think this scene rivals the "I'm with you now Pop" scene in the hospital in showing Mike's transition.