If Greg has a scene with Nick and Sara, even stemming back to season one, and later when he had his “crush” on Sara, his attention’s on Nick. Nick’s the one he wants to talk to, the one he wants to impress. Plus, they stare at each other, and seem to ignore others in the room when they’re talking.
Not to mention, Nick’s attention is on Greg as well.
I always, always, always felt that Sara treated Greg like a little brother. She helps him, tries to make him feel better, and jokes around with him. I think I know how she feels first hand, I have three little brothers. The similarities are perfect. She has a great relationship with him, not romantic, just in general.
Though, when he’s with Nick, there’s tension through the roof. Sexual tension, romantic tension, the works. He tries to impress Nick to no end, and sometimes it seems like he’s desperate for Nick’s attention and admiration. “Random Acts of Violence” is a great example. Same with “Scuba Doobie Doo” is a good example of Greg trying to impress Nick with his knowledge and jock-like accomplishments.
You can tell they really care about each other as well. They talk to each other about their personal lives and seem generally interested each others. Of course, Greg looks up to Nick. I believe he truly wants to hear Nick’s opinion and see Nick’s way of doing things.
How anyone can not see at least the major subtext is beyond me.