I think it's an adverb bit don't completely trust me
The development, the happening, the incident, the occurrence, the situation. The affair, the encounter, the meeting, the moment. the episode, the problem and solution. Others have mentioned these or their ilk: the chapter, the passage, the story, the period, the attack, the bout, the installment, the phase.
Because it gives the impression you have a poor vocabulary.
No that's not quite correct Grammar. It should be more along the lines of "He is very serious like that every time he's listening to music on YouTube. He is a certified music lover at a young age."
