It is amazing how the 2nd tent scene has affected us so much, but I agree, I think it is probably one of the most romantic love scenes I've ever seen. I think, for me, it comes down to the chemistry between Ennis and Jack, an unspoken chemistry up until this point, and, the way Ennis is, so vulnerable, so scared, but so wanting to be with Jack, but unsure of what to do, how to be. And when Jack puts his hand on Ennis's arm and looks into his eyes, forcing Ennis to look at him - that just makes me swoon. And then when Jack touches Ennis's face and kisses him - it's so beautiful and natural and I never want it to stop! God, I'm having trouble concentrating on anything, especially when we get talking about this scene! I know there are many other amazing and fantastic scenes in this film, but somehow this one transends because of it shows us the true nature of their love finally being realised. Notice how firmly Ennis keeps hold of Jack when they lie down - it's like, for him, this is it, this is real, and I want this, and I don't want to ever let you go. Heartbreaking and gorgeous all at the same time.
I also wonder if this scene was rehearsed or if they just went with the flow? I'd be interested to hear what others think about that.