Why is Will shocked by Tris' tattoo?

Will is surprised that Tris got a second tattoo because he believed her more sensible.

“I can’t believe you got another tattoo,” he says, shaking his head.

“Why?” I say. “Because I’m a Stiff?”

“No. Because you’re…sensible.” He smiles. His teeth are white and straight. “So, what was your fear today, Tris?”