IMHO - and I am not very experienced is that this is a macho thing.

The reason it's hard to lay down the 2nd Nuts is that you can't bear being bluffed (or even semi bluffed) out of a pot. The same feeling that you get when you fold KQos in early position and the flop is KQx. That was YOUR pot and you surrendered faster than the French Army.

Let's say it's your K-flush vs. 3 suited board, and you are rasied over the top.
There are 3 possibilities:

1) He has the AXs
2) He has XXs
3) He has a pair or 2-pair (semi-bluff, if he thinks you have the flush)
3) Pure bluff.

You win 3 out of 4 - but are the odds greater than 1/4 that he has the A? If so, you should fold (modulate pot size/bet/etc.). Obviously - it's impossible to call if you don't have a read on him. PFR from a non-maniac not shorthanded... he probably has the A.

Maybe he will get cocky and show you his bluff.... maybe he will think you a weak, passive player. Make him pay for that later.

Of course, if you're pot commited... it's irrelvant, although if you are and he IS bluffing, then he's a total moron.