Why am I so nice at first, but then get super bitchy when I'm comfortable with someone?

I'm almost always super nice to people I don't know very well, but then I start to get passive aggressive and critical of others? I want to be a good person, and I will make consistent efforts to improve myself. I don't know where this comes from though, it's really weird and sucky. So, maybe if I know why, it'll be easier to improve that aspect.

Why am I so nice at first, but then get super bitchy when I'm comfortable with someone?
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