Sometimes when I'm riding getting passed reenergizes me and makes me want to push a little harder to keep up with the othe guy... I'm usually an over friendly rider though so I'm the type that is you pass me I could speed up to stick with you but I'll usually say a little "nice bike, mind if I try'n keep up for while" or something like that... I've seen really bad whipe outs riding my road bike when people pull up and try'n "draft" someone without letting them know, so I always verbally let some one know I'm there and usually try'n make it clear I'm just try'n to push myself a little, I'm in no way offended that people occasionlly pass me.

IMO it's pretty rude(not to mention dangerous) to ride close to someone without letting them know. And I would say something to that effect if someone rode up on my rear stayed there!