No matter how mild-mannered a young dragon typically is, they’re prone to moody swings as they grow and it’s not unusual for a teenage dragon to suddenly become rather grumpy.
They can’t help the sudden sour mood. Of course, bad behaviour should be corrected, but no amount of telling them to “cheer up” will change anything, so the best course of action is to let the mood pass at its own pace. Often they just want to be quiet and still, and they can actually be rather productive in their bad moods, simply because they’re keen to get a job done and out the way.
As quickly as the bad mood comes it also goes, and the dragon is back to its usual temper. Many will even apologise for the sudden change, as they don’t enjoy it either.