[PorkchopDJG]

There may a few instances that I don't raise with a big pair before the flop. These are all limit holdem I don't know how or if it would change in no limit.
1) It's a super loose/passive type game and I have QQ or JJ in the SB or BB and 5 people have already limped in.
EXPLANATION: You raise to make people fold or build the pot if you raise here it's likely everyone else will call anyway then when the flop comes without a Q but with an A or K you are dead. Even without an A or K it is somewhat likely that someone may have caught two pair, trips or a huge draw in this multiway pot. With AA and KK I'm still probably raising from the blinds but with AK,QQ and JJ I don't really see the point. From early position yes because some may fold but from the blind no.
2) I'm in a loose/aggressive or tight/aggressive game and I have AA or KK in the BB and someone open raises from late position.
EXPLANATION:From the SB you would raise for sure to isolate the raiser and lock out the BB from calling but from the big blind you sometimes may 3bet it but you could also just call hoping to disguise your hand and raise after the aggressive player bets the flop.
3) I'm in a loose/aggressive or tight/aggressive game and I haven't played a hand in a long time and it gets folded to me in late position and I have AA I may just limp.
EXPLANATION- The aggressive guys will see this as pure weekness and raise it and maybe if someone has something like AK they threebet it allowing me to cap it when it gets back to me.

So I would say in general it's best to raise with all big pairs before the flop but there may be some circumstances when you can mix it up and play it a little differently to throw people off.