Buying an Extra One

After I started dating her, I noticed she had a strange habit. Every time she bought drinks, she'd always buy an extra one. "Can you even finish two cups?" "No, I can't." "Then why buy two?" She'd just smile and never answer. One day, I deliberately didn't go to the convenience store with her. When I got back, she was still holding two drinks. "You bought two even when you were alone?" "Yeah." "Why?" She looked down at one of the cups. "Because it used to be two." I was taken aback for a moment. Turns out, before she met me, she often bought drinks with her younger sister. Later, her sister went away to study, but she never broke the habit. I didn't say anything and just reached out to take one of the cups. "I'll drink this one from now on." She looked up at me. "You don't mind?" "Not at all." After that, every time she bought drinks, I'd deliberately ask: "Still two today?" She'd always smile and reply: "Yep." There was one time when I was away on a business trip for three days. When I returned, she came to pick me up at the station. She was holding two drinks in her hand, as usual. I smiled and took one of them. "Did you already know I was coming back?" She shook her head. "No." "Then why did you buy two?" She pushed the other cup into my hand. "Because I'm already used to it." I looked at her. She then softly added: "Used to having you around."