The other day I came up to a rural T junction and I was turning left, I saw just up the road a rider, with drops... I casually upped the speed as I "didn't want to be overtaken", if they are fast I don't mind but it can be a little embarassing to be passed.
Another time I followed a more hardcore looking rider down a descent they were takin g abit slow, it was a bit twisty so I held off passing until the bottom, of which I did... but they passed me again later on - I wasn't trying to pass them early on but they were going at it a bit slow, felt a bit stupid though being repassed.
Some time back I passed a person I see often, on a soft hybrid with rack and rack bag, I was tired to hell at the end of a 40 mile, wanted to pass them on a climb but my speed must have only been 1mph more than them... but I didn't want to take a million hours to pass them so I picked up a bit of speed - this killed me, and I was passed just after the top, what annoyed me is they came within a foot of me, said my light was on - scaring me out of my skin. I took it easyish for a while with intent to take them again on the flat but it just wasn't happening (and he turned off too soon.... chicken shoot

).
Sometimes I do ease up behind someone I am slowly catching up, just so I have the energy to pass quickly and gain a bit of distance before returning to my rythmn.