After Jack started bitching about having to stay in the pup tent with the sheep and after they agree to trade places, Jack says as Ennis is leaving for the first night with the sheep: You won't get much sleep, I'll tell you that.'
The way that Jack tries to lock eyes with Ennis mounting the horse. , I get the sense that Jack REALLY doesnt want Ennis to leave because he is really horny. Notice the way Jack walks, and looks at Ennis with his bedroom eyes.
I haven't thought about it before! well, when Jack looks after Ennis when he goes I've noticed this kind of yearning in his eyes, he was obviously sorry that Ennis had to go... what I didn't realise, and now it seems so obvious to me, is that by saying "you won't get much sleep, I'll tell you that" he's actually trying to convince Ennis to stay with him in the camp. and all the bitching avout Aguirre was because he wanted to sleep at the camp with Ennis, to be with him. it makes perfect sense now.
Also notice that at the FNIT scene he was striving towards it from a very early stage. He was the one who told Ennis "you're barely standing on your feet, it's too late to go up to the sheep", and when Ennis decided to sleep by the fire Jack told him that he would freeze to death, and if I remember correctly he added
"you're better off sleeping in the tent"... he was doing all he could to make it happen.