"Joe and I really love sad songs. We've always bonded over music," Swift told Zane Lowe on Apple Music. "We write the saddest [songs]. We just really love sad songs. What can I say?..It was a surprise that we started writing together. But in a way, it wasn't because we have always bonded over music and had the same musical tastes."
"He's always the person who's showing me songs by artists and then they become my favorite songs," she added.
"'Champagne Problems,' that was one of my favorite bridges to write. I really love a bridge where you tell the full story in the bridge," the multi-GRAMMY winner expressed, before diving into their song "Exile."
"He's always just playing instruments and he doesn't do it in a strategic, 'I'm writing a song right now' thing. He's always done that," she said of her boyfriend's musical abilities. "But do I think we would have taken the step of, 'Hey, let's see if there's a song in here. Let's write a song together?' If we hadn't been in lockdown, I don't think that would have happened, but I'm so glad that it did. We're so proud of that one."
Swift shared how she remembered the exact moment that she "walked in and he was playing that exact piano part."
"And all I had to do was follow the piano melody with the verse melody. So because the vocal melody is exactly the same, pretty much it's mirrored with the piano part that he wrote," she explained of their collab. "And we did the same thing with 'Evermore,' where I'll just kind of hear what he's doing and it's exactly, it's all there. All I have to do is dream up some lyrics and come up with some gut-wrenching, heart-shattering story to write with him."
The pair has definitely gotten a lot closer amid the pandemic. A source told ET earlier this month that Alwyn has been a "constant" for Swift amid a tumultuous year -- which has included not just the coronavirus lockdown, but also her mother Andrea's health battle.
Meanwhile, Swift told Lowe that she has "absolutely no idea" what's next for her, adding that while she's always been a planner and list-maker, "I think my new list will be places I want to see in the world, adventures I want to have, experiences I want to have. Things I want to learn."
"I think that'll be what the list looks like, because there's always going to be a list. It's me," she relayed. "But it will be lists of... In quarantine, my lists was... I decided to start trying to cook everything that I had always loved to eat, but never been able to cook. So there's always a list."