Why do we use "at" in the phrase "good at [doing]"
Preposition at is often used in the following context:
to show the activity in which someone's ability is being judged:
- I was never very good at sports. He's very good at getting on with people. She's hopeless at organizing things.
but also
to show the cause of something, especially a feeling:
- We were surprised at the news. I was quite excited at the prospect. Why does no one ever laugh at my jokes?
(Cambridge Dictionary)