Why did Desmond offer to go in Charlies place?

Desmond was rather heroic in the hatch when he used the failsafe. Other than saving Charlie's life all this time he hasn't done much to continue his heroic act in the hatch. I believe he mostly has been saving Charlie just because he has no choice, he's not saving Charlie, but his conscience can't let him die when knowing it's coming. So the question is, why be a hero now? Desmond saw flashes of events that led them to The Looking Glass but he didn't reveal everything he in fact saw.

What was it exactly Desmond saw? Two or more rather attractive females in a underwater hatch. Desmond didn't want to miss this opportunity. This brings me back to the vision Locke had in "Further Instructions" with Boone. This vision keeps ringing true to future events, it's not just to direct Locke to help Eko.


"Forget it, he's helping himself" Boone says to Locke as Desmond walks by with several attractive flight attendants. Unfortunately he didn't see Charlie whacking him over the head with the paddle in his flashes. Shame on Charlie for denying Desmond of this great moment. You already had yours Charlie, let Desmond have his!