[St_Thomas_Aquinas]

No matter the emotional coloring of Senator Cruz's outward rhetoric, his mouth typically tightens into the same straight line.

That is the answer. Cruz has control over his emotions because he is governed by reason. It's a sign of sobriety and composure to a degree that most people aren't accustomed to.

[Stentor]

It's a sign of sobriety and composure to a degree that most people aren't accustomed to.

You need to write a catechism for Ted.