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...
The fact that you're so stupidly 
quick to laugh all the time...
حقيقة أنّك سريع التأثّر لدرجة
...غبيّة وتضحك طوال الوقت
Dass du …
El que siempre puedas reírte
por todo como tonto…
El que siempre puedas reírte
por todo como un idiota…
T’es comme ça.
Souma,
Também, você começa a rir
feito idiota por qualquer coisa...
Сома, ты такой чувствительный,
вечно ржёшь...