Tris isn't the only one who wants to express her individual choices against the faction norms (see above). Even Caleb, the perfect Abnegation, does something very non-Abnegation here. And we know this is a big issue because Tris keeps breaking in to tell us it's a big issue: Caleb's eyes are stern, his voice has an edge. If this were just a boring line of dialogue, Tris probably wouldn't be noticing all these little physical clues.