The physical sensation of the hunger itself probably feels the same, whether you choose to be hungry or not, but in the case that you choose to do it, you must probably think of yourself as unimportant compared to the greater good (as defined by you), as "expendable." That goes against your survival instinct, which is programmed to look out for your own survival (also for a greater good: the survival of your whole species). So I think it must take a lot of mental strength to get to the point where you can do something as extreme as starve yourself to death. There are also other situations where people let themselves go hungry, but to a much lesser degree, e.g. when trying to lose weight. The fact that it is so uncomfortable to make your body adjust to a lesser food intake is one reason why a lot of diets fail. The motive is high (better health, beach body, etc.), but hard to do, because your will alone does not have anywhere near as much power as your survival instinct.