"He got sad" and "he became sad" sound close in English. In Spanish they need two different verbs, and the question that picks one is how long the change lasts. Se puso triste. He got sad (this afternoon, he'll be fine tomorrow). Se volvió triste. He turned into a sad person. Ponerse is a passing state: mood, nerves, going red. Volverse is a change in who someone is, and usually one nobody chose. Same adjective, and the verb alone tells you if it was a bad day or a bad few years.