That's a good question. I think he should be more aware that he is being manipulated. I started long ago with his mother and Aunt lying to him.

When Juliette found out Nick was a Grimm, she slipped him a sedative and took him to her veterinarian clinic for a neutering.

@wonder2wonder said:

Though Nick is the main character and the hero in this TV show, he is easily manipulated by the women in his life. He cares: is that his weakness or his strength?

I don't think he's being manipulated or that he allows himself to be.

As far as caring for someone, no matter who or what you are...caring will always be a weakness. However, it is a weakness that you can draw strength from.

If someone threatens a loved one, you can find strength in doing what's needed by any means necessary to help the person you love.

So, the long and short to your question is...it's both a strength and weakness. But a weakness that can be used to one's advantage.