Once you are assigned to a flight, the whole crew is assigned at the same time, and then that crew trains together for a whole year to prepare for that flight.
So most astronauts getting ready to lift off are excited and very anxious and worried about that explosion - because if something goes wrong in the first seconds of launch, there's not very much you can do.
The pressure suit helps if something goes wrong during launch or re-entry - astronauts have a way to parachute off the shuttle. The suits protect you from loss of pressure in case of emergency.