[Robbie Williams]

There are some very valid reasons someone might flash at the person in front of them:

• "Hey, I'm a lap ahead of you, please make room at your earliest convenience."
  
• "Hey, I have a run on you and you might not have seen me."
  
• "Hey, it's quali and you're just coming out of the pits and I'm on a hot lap, so I'd appreciate it if you don't pull out in front of me and ruin that."
  

and then you'll see this:

• "HEY YOU JERK WHY DID YOU CUT ME OFF?"
  
• "HEY LOOK AT MY LIGHTS DON'T GET DISTRACTED TROLOLOLOL"
  

With experience you learn to accommodate the first list as best you can and ignore the second list whenever it happens. 
(Two years later and I'm still learning, too!   )

Yeah, I agree that the second list you just have to learn to deal with and the first list is completely acceptable. Just annoys me that some drivers do it purpously to distract you in the braking zone to try to force a mistake. Seems childish to me lol. But yeah, I suppose it's just part of racing!