I'm really not angry. I just look that way. Today is a happy day.
It's cold. The sun is shining in my eyes. This limping thing I'm doing isn't just to get sympathy; I'm actually in pain. And I'm trying to read that sign over there, but I'm blind and the words are too small. Hence the narrowed eyes, the furrowed brow, and the general expression of displeasure. Besides, if you were attempting to retain the insane amount of relatively pointless information that I am, you'd be looking pretty grumpy too.
I realize that I might look like a scary person who just decided to murder someone, but I hope this post assuages your fear.
P.S. - Thank you, scrawny-looking freshman, for opening that door for me. I realize the action was motivated more by terror than chivalry, but I won't hold that against you.