If you are close enough to get into their Slipstream, you can essentially kill their drag. This might feel like a boost because the wind that was previously pulling them back is now transferred to YOUR rear and not sucking on them anymore.
Here is a photo of a bullet's slipstream (wake), where you can see the air sucking the bullet backwards.
There is no "physical" reason for someone to be mad that you are following so close behind them, in fact, the physics prove the reverse, unless they want that drag to help them train like how runners will use a parachute. Other than that, you might consider the stopping distance in an emergency, "personal space", and swapping the lead as courtesy reasons for following or not.