Souma is really susceptible to cracking up, and even memories will put him into laughing fits. But being self-conscious about this, he always tries to hide it. When he does this again at a clothing store his friend works at, the friend asks him why he does that, especially when it's so easy to tell anyway. Later, Shun also repeats the same sentiment...
That hardly sounds sincere
 when you're laughing too!
كيف لك أن تكون غير صادق هكذا؟
Du lachst doch selbst!
Pero si tú mismo te estás riendo.
Pero si tú mismo te estás riendo.
Tu joues très mal le gars vexé !
Non sei per niente credibile!
E você nem disfarça!
Да ты же сам стоишь угораешь!