Originally posted by darthmunky:

"...why is there such a build up to that? It seems like its supposed to be really epic."

It's the final punctuation for the series of dialogue leading up to it. The rebels have debated up to this point whether or not there is anything more they can currently do for Commander Skywalker and Captain Solo. They finaly decide that they have to think of what is most important, which means leaving Luke and Han's survival up to themselves. This is a huge, heartbreaking moment for the characters involved and, just to make sure the audience knows "Yes, we are going to leave Luke and Han all alone" we have the dramatic closing of the blast doors sequence. It leaves us wondering if there really is any hope for our two heroes and makes the revelation that they did, in fact, survive the night all the more satysfying.