As a child your parents did anything they could to torment you, drinking, drugging you, abusing you, raping you, etc, but the thing that really stuck with you is the... isolation. No they didn't lock you in a dark room alone... often... the way they isolated you is by getting you attached to animals and... and then torturing them to death in front of you. Because of that you closed yourself off, not allowing yourself to get attached, for both theirs and your sake.
As much as it doesn't make sense to worry for someone like TF141 you couldn't get over your fear of attachment, so you kept yourself distanced, but making relationships with humans scares you a lot less than making relationships with animals. You love animals, you do, always have, and that's why you don't allow yourself to be near them, because if you do, and you get attached... who knows what'll happen to them?
One day in the mess hall, as you and the rest of TF141 are eating, you feel something nudge your hand. Looking down to see what it is, you find Riley sitting at your feet. You tense automatically and quickly walk away.
Riley, a german shepherd that's with the team, doesn't let it go, for whatever reason he becomes determined to get your affection, and follows you throughout the base day by day. As much as you enjoy the obvious care and affection, every time you even touch Riley pictures of your old pets bloodied corpses flash in your eyes, reminding you why you can't allow yourself to cave.