I start fires and hot romances, what am I?

When I leave, the walls grow teeth, And whispers speak where silence stood. I'm what the madman mourns beneath The laughter no one understood. Check me, keep me, lose the thread, I'm sound, but only in your head.

Step foot in me, you are six feet under. You may come to me when you hear whistles, sirens, and thunder.

Why was the guy who was hit in the head with the can of soda lucky?

What five-letter word, typed in all capital letters, can be read the same upside down?