I know I’m beautiful.
I know this because of my parents*.
My mom is beautiful. My dad is handsome. Since I have their genes, and many of their other not-so-visible but more wonderful qualities, I think that makes me good looking.
This logic works for my siblings too. Lori, Adrian and Danny are all beautiful/handsome.
*Going back to my grandparents and looking at my extended family, I’d have more evidence to support my conclusion.