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せっかく sekkaku
After all that effort — don't waste it.
What does sekkaku mean?
The word that marks something as hard-won or rare, so letting it go to waste would hurt. It powers both sulking ("after I made it!") and persuasion ("since we're already here, let's go").
Example
せっかく作ったのに、誰も食べてくれない。
I went to all that trouble cooking, and nobody's eating it.
Vibe
Who you can say this to, and how it lands.
Safe to use Fine in ordinary conversation.
The story behind sekkaku
English needs a whole clause for sekkaku: it flags that effort, luck or a rare occasion is sitting on the table, and that wasting it would be a small tragedy. Follow it with noni and you get a sigh of wounded pride; follow it with dakara and it becomes the most effective invitation in Japanese, the one that gets tired people to walk to the second restaurant.
The kanji literally read "break a corner," and explanations trace back to classical Chinese anecdotes, though which one is genuinely debated. The feeling, at least, is unmistakable.