The strange thing is, I can't see Nick with anyone except Greg. Don't know why, I should probably be able to see them both in different relationships
You see I see the complete opposite.
Nick has always had a flirtatious manner with Catherine, Mandy and Sara. Remember in Rashomona when she was holding some up lingerie and she said, "I need your hands." and Nick said, "I thought you'd never ask." Then Catherine made a remark about being being careful and Nick said, "Cover your ass?" and she said, "Yeah." and he said, "Loved too." He's always been very flirty with the ladies. Mandy and he have the cutest of flirty scenes starting with Mandy Dandy in Burked and then onto "Oh Mandy" in Happenstance.
However, with Greg, he's much more of a brother to him. Much more of leader. I've never seen anything that indicates there's a romantic stuff between them. If anything Nick finds Greg very annoying at times.
Greg and Riley have flirted a few times together and Greg had a huge crush on Sara for eons. We kind of got that inkling in Fannysmackin' when he told Sara he'd know her 'scent anywhere.' That's indicative of a crush. He hit on her plain and clear in the very first season.
But everyone sees what they want to see, even I. And I certainly hope I don't get flamed for this. After all, the writers' in the end will do what they want to do with the show, it doens't matter how we feel or who we hope the characters wind up with. TPTB will do what they want and how they see fit with the show.
And someone can point out the whole argument about subtext, but subtext is subjective. It's not a science. I never saw any subtext with GSR because it was a well known fact Grissom was going to wind up with Sara, it was a question of when it would happen. It happened to take too long for it be canon, but it was there. The arc was very cleary. TPTB simply teased the audience for eons about where things stood with Sara and Grissom.
As for Nick and Greg, there have never been any clear cut signs saying there was more to their friendship than it was. Nick's a bit more obvious with his emotions, so I think it would have been much clearer if there was a romantic relationship because of how Nick is with his emotions. He's an open window.
I know there was some talk of him being 'jealou' in Leave Out All The Rest' when he snapped at Riley and Greg, but they were pushing the professional boundary with their sex talk and he told them to back off for each other's own protection, there was no jealousy there at all.