てしまう when there's nothing bad happening?

Question

I always learned てしまう = 'oops, did something regrettable', but my exchange partner said 「桜が全部咲いてしまった」about spring blossoms, which sounds like a nice thing? Did I misunderstand the grammar?

Accepted Answer

You're right that 'regret' is the most-taught nuance, but the core meaning is really 'completion, often unexpectedly fast or total, with an emotional charge attached' — that charge can be regret, but it can also be surprise, or even wistfulness like your example. 「桜が全部咲いてしまった」carries a nuance of 'wow, already all bloomed' — maybe a little bittersweet because it means spring is passing quickly, not literally sad. So the emotion is context-dependent; 'completeness/finality that hits the speaker' is the more accurate core than strictly 'bad thing happened'.