Why Do People On The "Outside" Get Mad About Inside Jokes?

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Me and my friends have a lot of inside jokes. But I've noticed that a lot of times, people that don't know us get mad at us for having inside jokes. Me and one of my friends were in Geography, and he kept saying "This is how you make beef jerky taste like sweet corn." and we'd laugh. Then some girl asked him what he was saying and he said "This is how you make beef jerky taste like sweet corn" and we both started laughing and the girl said "That's not funny!" and I said "It's funny to me" and she got all red and angry. I thought the purpose of an inside joke was that it was just something between friends. I'm sure that everyone else has little inside jokes that only them and their friends know about. What's the big deal?