It seems very obvious to me what's going on here.
First of all: Love it. Combining history, tune and this game is, considering the Escapists flair, quite a feat. Now, moving on to my interpretation.
The beginning gives it away easily: It is about the Spartans, and their feelings. These fictional soldiers of awesome are human, and they have feelings. How does a person feel that is told that he and 200 others are the best of the best of the best of humanity? They are sent out against terrible odds, and expected to win regardless. They get thrown against waves and waves of enemies, as shields of humanity, as heroes of their days - unsung heroes, reduced to tools of war.
They get morally lost, deprived of the simplicity of life. Spartans had no childhood worth mentioning. No lovers, no friens - their life is war and training. Now, why them? Who are they to be augmented to these levels, and why are ODSTs or other soldiers not? It is the age-old question; why me? We are all meant to shine - as children do. Spartans have no childhood. They were trained rock solid at young age, taken away from family and social surroundings. Their "mother" was a scientist, and their "playground" was a boot camp.
Yet, all these abandoned things aside, they are the essence of warfare, supersoldiers, meant to surpass all military expectations. Someone has to safe the world - wouldn't that task shatter your mind? I would be frightened by the power to do so. Aren't we just mere mortals? How can a single being have so much impact? Perseverance, of unbelievable levels. But they need this mentality. The Spartans need to see themselves as heroes, they need to believe in their power, as mind warping as it might be. If a Spartan loses that trust in himself, he is shrinking, and that helps no one. In fact, a single Spartan breaking under the psychological pressure is a loss of decades, millions and potentially fatal for humanity.
The speech fits spot-on.
I am thankful for this work.