Whats up with adults not letting you pay?



So as a teenager I don't really have a source of income but I still don't like relying on anyone but family. If I'm out I don't like other people paying for me and I prefer to pay for myself. If I don't have money on me I won't get anything or I'll get something super small. Today I was with my partner and found myself in a small family outing and they asked if I wanted to join for dinner and I said yes. I got something small as I wasn't really hungry and at the end I gave the people paying the money for my portion of the meal but they seemed almost mad and kept telling me to take it back so I caved.

Tldr: Casual dinner with partners family and they didn't let me pay for myself

Why are they so insistent and almost angry? and is it rude of me to want to pay for myself in that situation?

Whats up with adults not letting you pay?
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