The nuances of sexual attraction are so well portayed here. Jeremy is a tough guy, as his army experience testifies, yet his sexual nature is shy almost like a girl! I think Nick actually finds this innocence and reticence appealing (particularly after the brutality he has suffered one way and another) and these are qualities that Jeremy will always retain to some extent.
These qualities in Jeremy bring out a protective and respectful response in Nick which aligns with his more dominant and aggressive nature. In fact only a such a man as this could bring out Jeremy's latent passionate nature. Jeremy could only be aroused from his passivity by specific acts and a specific kind of partner. Just as with Ennis and Jack, one is the physical top and the other is the emotional top.
The evolving dynamics of the relationship is well expressed by the fact that Jeremy is now "sweet thing" and it is Nick who is now the "stud"!