What are reasons why people will flash high-beams at you?
Obvious answers - your high-beams are on, your lights are off (if car is facing you), or if they are particularly rude, that they want you to go faster (if car is behind). But what other reasons don't I know?
The situation: I'm driving home and it's dark and rainy. I'm going at or just under the speed limit (due to weather and it being a curvy road with low vis). The car behind me occasionally flashes its high beams - on for a second or two and then off again. This happened several times. I checked to be sure my lights were on, my wipers were on, and my speed (they were not tail-gating, in fact were keeping a bit back). As I kept driving, a car coming towards me flashed its high-beams too. I wondered if this was due to me or because the car behind me was flashing them, and I checked everything again. Finally, I pulled over, the car passed me, and I didn't see anything wrong (tires, wipers, lights, nothing open, etc.). I drove home, checked my car again, and nothing. What was going on?