For some it might actually be because they no longer want to go on here. But for too many recent casualties it's just a way for them to grab a last bit of attention. I don't understand why deactivating is necessary if you're just going to fade away anyway.
Probably because she was bored and did not need it anymore. I would not deactivate my account even if I had a girlfriend because I like this site and give advice and answer questions. This site in my opinion is better than Facebook.