I've been writing letters back and forth with a past teacher of mine. She used to start every letter with "Dear [my name]". But lately no matter what I wrote, she replys by starting with "Hi [my name]" or "Hello [my name]". So do people express their subtle messages, like they don't want to correspond any more, or they're glad to reply, by changing the starting words? Thanks a lot.
Mmm... I think that "Dear" is a bit more polite than "Hi"/"Hello"... That could be why when you received the first letters she used to start with "Dear" and then she began to use "Hi"/"Hello"... However, I'm waiting for more posts as well since I'm not a native English speaker...
It's quite difficult to answer this question with the information you have provided. I used to correspond a lot (in English) before.Sometimes I wrote the greeting without even thinking much, since my mind would be concentrated more on the body of the letter. I also concentrate on the ending of a letter but that's just me.

If she is really not interested anymore, the body of the letter should reveal far more than the initial greeting. The sentences will be brisker with not much explanations, not much punctuations either... And she may not describe too many events like she used to... So I guess your answer lies in the next paragraphs.

Sometimes it also happens that when we are so close, we don't need to show too much respect anymore, we sort of take the other person for granted isn't it?

But if you're really sure something's wrong, maybe she is going through a hard phase in her life? How about asking her if things are alright(if you are both really close)?