Jill Pantozzi explains:

Gwendoline Christie did an interview with TV Guide recently where she said some interesting things about gender roles in Westeros. 

“Brienne has seen this woman exhibiting strength in an intellectual manner with Renly, and also exhibiting strength of love and motherhood that I think she sees as equal to her own physical strength,” Christie said of the character Lady Catelyn Stark. “And perhaps it’s the first time we see Brienne consider something beyond the strength of the physical in a woman as a means to be equal to a man.”

Christie also mentions the respect and love Brienne has for Renly but also for Catelyn, even though she represents everything she never wanted to be, i.e a mother. “She’d rather die in what she sees as a noble death rather than potentially die a conventional one that she feels is without much merit for her,” she said. And then they asked her how cutting her hair short for the role affected her in real life and she gave a really great answer:

I struggled for a long time with my hair, but then I’m grateful for the opportunity to realize that femininity doesn’t have to come from hair or any of those traditional female archetypes of appearance, So, that’s been exciting actually. I can’t speak with any kind of authority whatsoever because I’m just an actor and I only have my opinions, but I do think it’s really refreshing to have a woman depicted on a mainstream TV show that doesn’t obey typical aesthetics of females and the way they have been portrayed in the past. And I’m really excited to be portraying one of those women. And I hope that her popularity signals a greater expansion of people’s views about men and women and that gender types can be more flexible.