Giuntoli has indicated that Juliette represents Nick's past life and with her gone he's pissed that his old life is gone. He's now going rogue. http://www.nbc.com/grimm/video/season-of-the-hexenbiest-david-giuntoli/1423717/
So it seems for Nick to get totally committed to the Grimm persona he's going to have to give up Juliette. And to prepare us for that break the writers are making her so wooden.
I don't think the writers ever wanted us to get emotionally invested inher character. That was the problem in the TV series Angel which creator David Greenwalt worked on. Angel was tied down by the "soulmates" mythos with Buffy so that many fans would not accept him in any other relationship and that limited the writers ability to create new sizzling storylines and the series eventually cancelled.
Juliette is the only character in the cast who is written at arms length. We're told nothing significant about her past, her goals, her character. Tulloch is a great actress so I think's she's being directed to act reserved. Nick shows passion, love, emotion. We're suppose to get attached to him. Juliette I think we are suppose to feel nostalgia for but never want her back in Nick's life. I hate that but I think it isn't the "true love that lasts through all time" mythos that Grimm is shooting for. I have to go over to Once Upon A Time for that fix.
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