Hail Venus, it's a nice post.
Although I know yours was a general post I feel it applies specially to me.
I can't talk for the others, but in my case, being a fan of a tennis player isn't something pasional, but rather I follow a player because she plays the way I like, and I like watching her. Of course, it's not that simple, I do feel happy when Justine or Martina win, but my support for them is not something I have under my skin.
Maybe it is with Justine, because the circumstances were quite different.
I become a fan of Martina after seeing her win the Australian Open for the first time. It wasn't that I started following her because she was winning, but I discovered her game there. Being a long time fan of Gabriela Sabatini and Jana Novotna, it was gratifying to cheer for a player that actually was winning everything. But there wasn't a strong bond, I knew her as a champion and somehow she spoiled me, in 1997 she didn't lose a match until RG, and that was after a long lay off, then she lost only 4 more matches, and started 98 winning the Australian Open again, so, I guess deep inside me I expected her to keep doing it.
Justine is different, I started to follow her when she was nobody. I've seen her progress, and every aspect in her game fits what I like, plus, I admire how she overcome adversity, she is a player that I can admire as a person, as well as a player (not that Martina is a bad person or anything, simply that Martina is like nearly all the other tennis players, there isn't anything special about her life).
So, it's something that I can explain, but simply it's the way I feel. When Justine plays, I am nervous, I follow the live-scores and I am swearing all the time, I dedicate time to the website, I feel really happy when she does well, and I am looking forward to her next tournament expecting her to break through, with Martina I am now much more objective. I do like watching her play, I do like her winning, but there isn't passion. It could be unfair on her, but it's the way I feel, I can't help it.