No one can predict the future. Nick was in love with Juliette, in the first ten minutes of episode he was buying a ring to propose to Juliette after he got home. He didn't know he was going to shoot someone in a couple of hours--that kind of spoiled the mood. You can tell a teenager not to start smoking, they won't listen, soon they are addicted. After the cat scratch he tried to explain that Adeline was a real witch. Honestly, would anyone believe that? No one warned them about the hat or Juliette being transformed into a hex. Would anyone give up their beloved because a dying relative said to do that? Tell your significant other you would drop them like a dead rat based on the advice of an aunt--go ahead but leave instructions for your family to let me know when your funeral is scheduled so I can attend. Of course, it was hard to ignore Grimm business, I would probably have gone to a shrink after seeing the first wessen.