*scritches head* I think, if I got this right (and while I'm no doctor I have heard various medical relatives argue about this before) that when the body is exposed to a constant amount of pain over a shortish period of time that is not accompanied by shock - not getting an arm slowly ripped off, for example - then the brain produces endorphines to help deal with it.
So essentially, on a level we rarely recognise, we actually enjoy it. It's why people eat really hot spicy food, it actually hurts but we get a kick out of the endorphines. I figure that in B.E a similar thing might happen, though I think I recall a few stories where it really did hurt until the process was done. The other half the time, when the process is magical etc... well magic has the 'and they shall know only pleasure!' switch :P
I hope your spurt becomes less painful for you, though I think telling people on this forum that your boobs are having a growth spurt will mean that a lot hope it doesn't stop for quite some time to come hehe
Out of interest, I know some girls who swelled up while on birth control, I never stopped to ask if that hurt too. I should have. Knowledge, anyone?