Strax really has no idea why the Doctor is so emotionally attached to humans. Strax has no idea why anyone is emotionally attached to anything at all. The strategic military endeavours of any Sontaran battlefleet could crush anything or anyone that dared to get in its way; there is, therefore, no reason to form emotional attachments in the first place. Any good soldier understands that one must forgo certain of the pleasantries of life in order to crush the worthless bodies of your enemies into the cold, unforgiving ground.
...the Doctor must not be a good soldier anymore. Perhaps the rumors he had heard about the man are false after all.
The Doctor is staring at him from across the room. His mouth is set in an ugly frown, and there is a great deal of pain in his eyes that Strax would normally revel in seeing. He does not revel in it now. Being captured by the enemy is not as thrilling an experience as he had hoped it would be.
"Commander Strax, is it?" the Doctor says darkly. He just stares, and Strax can see his mind working quickly to try and sort out what he wants to say. A very human thing to do; the Doctor must be spending too much time with them after all.
"If you are planning on conducting a strenuous interrogation," Strax says, "then I regret to inform you that it is not the Sontaran way to break under pressure. I will withstand any form of torture you may choose to implement. Sontar-ha!"
The anger drains from the other man's face, and Strax doesn't understand why. He has never seen this reaction from the enemy before. A Sontaran's firm resolution to remain steadfast in the face of anything and everything normally infuriates the enemy, inducing mistakes and slip-ups that a good soldier could make use of at a later date. The Doctor is looking at him as if he were the one being tortured, and it is most puzzling indeed.
"I'm not here to interrogate you," he states, his voice small and quiet. Not the voice of a soldier. "I just want to ask you one question, just one teeny-tiny question. You don't even have to answer me if you don't want to. You could even lie to me, I don't care, but if you do decide to lie," the man leans in close to him, so that he is looking straight into his eyes, and Strax can see the rage creeping back into him, "just know that I've been around long enough to understand how Sontarans operate."
"What is your question, Time Lord?"
The Doctor takes a deep breath and releases it slowly. He stands up and takes a few steps away. Turning one's back to the enemy is never a wise move. "Were those children your group gunned down innocents by the standards of the Sontarans?"
"They were not."
"Tell me why."
Strax hesitates. Hesitation is not a trait becoming a soldier. "They were the enemy," he answers.
"They were children, Strax."
"They came from enemy stock. Our orders were to destroy every worthless Trimestrain on sight. There were no restrictions to that order."
"They were children, Strax."
"If allowed to continue, they would have been combat-trained and sent to the front lines to destroy us. We obeyed the orders given us from our senior officers. It was wise to annihilate them before they became a greater threat."
The Doctor rounds on him, slamming his hands against the table and screaming in Strax's face. "They were children, Strax! Even by Sontaran standards, their hands were clean because they were too young to be recruited for battle! There was no reason to kill them, so don't you dare sit there and tell me that murdering those babies was a wise decision."
The Doctor runs a hand over his face in a vain effort to compose himself, and Strax is beginning to doubt that the Doctor was ever a good soldier at all. Strax says nothing. He feels as if he should be defending himself against the Doctor's allegations, but there is no reason to do so. Gallifeyans are weak creatures. They will never understand the risks one must take in combat.
"That's the problem with clone races," say the Doctor quietly. "You lack the evolutionary need to protect one's own genes. You don't know what it's like to have children, and you don't know what it feels like to want to protect them." There are tears in the Doctor's eyes. A soldier does not weep. "Well, let me tell you something, Strax: I had children. I know what it feels like to want to protect my children, and I failed to do it. I will not let bloodthirsty bastards like Sontarans run around killing innocent children who haven't done anyone any harm."
This confuses Strax. "You aren't my superior officer. You have no authority mete out punishment to me."
"You don't have a superior officer anymore, remember?"
Strax scoffs. "Ah yes, that's right. I am the sole survivor of my entire clone batch." He smiles smugly. "You murdered hundreds of Sontarans today, Doctor; what gives you the right to lecture me about dispatching six enemy offspring?"
He can see the Doctor's jaw clenching tight, and although he is normally very good at maintaining eye contact for long periods of time as intimidation, Strax is the one who looks away first. Perhaps he is not as good a soldier as he had thought. "From what I have heard of you, Doctor," he says slowly, thinking carefully about every word before saying it, "you do not enjoy engaging in warfare. The glorious deaths of my fellow warriors here today does not disturb me as they seem to do you."
"Talk about Captain Obvious."
"But upon further consideration, I see that you are correct about the deaths of the Trimestrain offspring. The honor of my clone batch has been tainted, and, as an upstanding soldier of the tenth Sontaran battle fleet, I wish to try to restore that honor."
The Doctor squints at him. "Why the sudden change of heart?" Strax says nothing. A soldier does not admit to weakness in front of the enemy. After a long silence, the Doctor seems to understand. His mouth turns up a bit at the corners, in a facial expression that the humans call "smirking," and he claps his hands together loudly. "Alright then, Strax, let's see here. What can you do to restore your honor?" He runs his fingers through his hair while he thinks, and soon, the "smirking" expression turns into a "smiling" one. The Doctor points at him. "What's the opposite of murder, Strax?"
The Doctor stills quickly before laughing. "I suppose you're right, but besides that. You know what I think it is?" The man whirls around in a circle, much more smugly than is strictly necessarily. "Protecting."
Strax's face falls. "No."
"Yes!" The Doctor laughs again, very loudly, and Strax is starting to fear for his sanity. Even the Daleks know that insane soldiers need to be locked up so they don't hurt any of their own. No wonder the last Time War ended the way that it did.
"Please, Doctor, please don't-"
"A nurse!" he yells triumphantly. This sort of emotional outburst is definitely unbecoming of a soldier. Strax is trying to keep his own frown at bay, but it doesn't seem like he's doing very well. "A nurse," the Doctor repeats, a bit more calmly this time. "You'll be a nurse, Strax, caring for the sick and weak and the children."
In due course, Strax is returned to his homeplanet, and the Doctor barges into the General's office to speak to him. Nearly an hour later, and after much yelling from both sides, the Doctor emerges from his office and disappears into his TARDIS, and the General motions him into his office. Needless to say, he was demoted effective immediately.
The next time he sees the Doctor, the man proves to him that he has relearned how to be a good soldier.