Nick was never meant to be the solitary Grimm. That was clear from the pilot. He would always have a SO and a team behind him. Plus, expecting him to give up his life after a couple of conversations with Aunt Marie would've been more unbelievable than what usually happens in the show.
As for giving Juliette up, I think it has been discussed in this forum how Nick was trying to have his cake and eat it too. He wanted to be a Grimm and he wanted to be with Juliette. It turned out to be a doomed relationship but he didn't want to believe that and that was the case even when she turned into a Hexenbiest. Juliette was the one that walked out. Nick has never been shown to break off a relationship the way Batman or Spider-Man have.
As for giving Juliette up, I think it has been discussed in this forum how Nick was trying to have his cake and eat it too. He wanted to be a Grimm and he wanted to be with Juliette. It turned out to be a doomed relationship but he didn't want to believe that and that was the case even when she turned into a Hexenbiest. Juliette was the one that walked out. Nick has never been shown to break off a relationship the way Batman or Spider-Man have.