There's a more detailed explanation of this concept in SSHE, but it has to do with "Protecting Your Hand". The raise is based a pot-size. Consider that middle pair, middle kicker is a 5-out drawing hand most of the time, which requires ~8:1 immediate pot odds to continue. If you are getting the odds to play, and your raise will face most or all of the field with calling two bets cold, then you should often raise. Sometimes you will have the best hand, and when you do, you want to ensure that nobody has correct odds to see the turn (this forces your opponents to decide between folding, or calling unprofitably, both of which are good outcomes for you). When you are behind, there are still a number of hands that you would prefer to fold, and could call profitably for one bet, but not two.

I suggest picking up a copy of SSHE, as there are a number of examples that are counterintuitive, with good explanations.