You don't expect friendships to last when you're a backpacker. People move on, it's inevitable, but they usually say goodbye before they go.
I met an English girl called Michele who was feeling really lonely. She was working with a bunch of European girls and was feeling very left out. When she found out that I was English and sticking around for a while too she was ecstatic.
She moved into the same room as me at the hostel so that she would have someone to talk to after work. She said she was sick of getting chatting to some one and then having them leave after a few days. I could sympathize with that.
She'd been in the room about 3 days when she got chatting to an English lad in a bar. The next morning she decided to quit her job and go up the coast with him.
She was gone the next day and didn't even say goodbye.
I'm not bothered that she left. I didn't actually like her that much. But after all that fuss it was rude to not even say goodbye.