Getting used to things is pretty easy in life, just like getting used by others. Pretty much it's the same thing. Either you get used by others or you get used to others.
However, getting used to being alone is a really terrifying experience. But the worst is when you get used to someone's presence in your life. Someone who wasn't there a few days ago. But that particular person still became a huge part of your life without you even realizing it.
One could say that Xiu got used to Darren's presence over the course of knowing him for a month or so. Perhaps that's why when she opened the door in the morning and didn't find him out there waiting for her, she felt something missing. She felt downhearted since she had to start the day without holding his hand again.