Joss Whedon said this quote
“ At the end of the day I have many answers for it. It has to do with my mom, who was an extraordinary woman, and a great feminist. It has to do with the people in my life. It has to do with a lot of different things, but -- I don't know! Because I'm not just writing from the female characters for other people. I have a desire to see them in our culture -- that was not met for most of my childhood. Except occasionally by James Cameron. [From the 2011 San Diego Comic Con, in response to being asked why he writes strong female characters.] ”