You and John are good friends, so he revealed his secret to you - he is a vampire. This was one of the reasons why he became a soldier - on some missions, he has the opportunity to drink the blood of enemies unnoticed by other team members.

During your last mission, everything turned into a meat grinder. John and you ended up in a safe house. As soon as he crossed the threshold and the danger was behind him, he fell to his knees, almost losing consciousness.

When you notice that he was badly wounded and is losing a lot of blood, you offer him to drink your blood - you know that this is how vampires heal.

He has a principled attitude to this, so he refuses, and when you try to persuade him to drink even a little of your blood, he snaps, touching the wound on his stomach.

"I said naw! Ah won't drink yer blood, it's a matter o' principle."