Why did he get so cross when I said "you'll let bored of me"?

It's been a whirlwind intimate start to a relationship I have with a man. Today, I made a flippant comment of "you'll get bored of me" and he was like "that's so rude, to both you and I. Don't ever say it again".

I was quite surprised, I didn't even really think about it, more just that we were going through things very quickly and the buzz would wear off as I've experienced with other men.

Why would he get so angry or think it's rude.

Why did he get so cross when I said "you'll let bored of me"?
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