"BECAUSE I SAID SO, THAT'S WHY!"
Anyone who has ever dealt with kids has probably said this at some point, or at least heard it when they were kids. I try very hard not to say it to my nieces and nephew because it's a total BS line.
I hated that one, and the truncated "because" or "just because". Then again, my favorite question was "why?" so I got this on from a lot of adults.
I hated kids and people who over use the word "like" in place of "um" in their pauses in speech. We had one girl in my grade talk like that from around third till tenth grade. Like was like every other word. It was like a total pain in the like ass to like listen to her like ramble on like that.