It is a common misconception that having been dealt Pocket Aces you are in a commanding lead and you are unbeatable, It is certainly a nice hand to start off with, but the Truth is there is no such thing, as a sure thing. Even if you hit a flop giving you 3 of a kind such as A,K,Q, You are still beaten by J/10 and the only way you can win in that situation is if you should hit the last Ace in the deck, or the board pairs up.
In some situations, even if you feel that you could possibly win with A/A, Folding to an all-in by a bigger stack, is always the best option. especially if you have smaller stacks still playing. let them go out, and even if you do fold you could gain a few places in a Tournament with other players making the same mistake on other tables.
Always remember Any 2 cards can win, and Any 2 cards can lose. That is why Doyle Brunson won 2 World Titles by calling an all-in from opponents with 10/2 in his hand. But he did have a massive chip lead over his opponents and losing those hands would not have cost him the Titles, had he lost.
I have on occasions called all-ins from other players without looking at my hand in Live games, knowing that I could not be busted out of the game if they beat me, and would still have a commanding lead over them.
In a heads up situation, you are always wanting your opponent all-in at some point and if you have the chip lead any 2 cards can beat them.