Why do some people act so very close and then disappear without even a bye?

This is more of a venting out than a question actually. Met this girl here. She was so stressed about her issues and I tried helping her, listening to her, suggesting her ways to come out of the situation. It was like 24*7 texting and the next thing I know now is she has left and no intimation whatsoever.
Also, if you got nothing nice to say then don't answer.
Why do some people act so very close and then disappear without even a bye?
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