Why is he observing me so closely?

We're, I'm free to say, best friends. He knows every little thing about me, my face, hair, expressions, when I feel said, happy, without me saying anything. Today when we saw eachother he started to teas me and he started to observe my face, saying that I blushed and then said it looks great with my blond hair and then again looking from my face to hair and it only made me feel even more shy.
Why is he observing me so closely?
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