Reading your comment, it makes me think that Bobby, Dean, Sam, and Cas are all kind of like Peter Pan and the Lost Boys waiting for Wendy to come and tell them stories and tuck them in. In some ways, they each try to fill the role on their own for the others. Dean's the only mom that Sam ever had and Jess was around for a little while. Bobby does some of the same for Dean and I think Lisa did also to some extent. Cas was adrift with only what he picked up from observation and the unreliable source of the Winchesters. Bobby is probably the only one that has any idea what having a female presence around is like but who knows his history other than Karen. I'm guessing he grew up with a mom. They try to compensate for the lack but when you see them with Ellen in the picture, you realize how off they've been. When Ellen was part of their lives, she wasn't diminished as a character. Still the same tough, competent woman she was but she had someone to share the road with and probably wasn't as lonesome. It made me miss her even more.