Warning: SPOILERS for To All the Boys: Always and Forever.
In To All the Boys: Always & Forever, Lara Jean (Lana Condor) and Peter (Noah Centineo) find the perfect song for their relationship in "Beginning Middle End". First, they comb through playlists and jukeboxes, resulting in many poor choices for them as a couple, but they finally come across their song in an unexpected moment. The song they choose, "Beginning Middle End," winds up being the perfect metaphor for their three-movie arc.
As their relationship started on false pretenses, Lara Jean laments not having the traditional firsts that most couples have, like a meet-cute or a song. The young couple decides they need to find their song before graduation, which proves difficult. Lara Jean ultimately finds their perfect song during their senior class trip to New York City, where she goes to a party without Peter. As she realizes she's falling in love with the city and makes the decision to go to NYU, a certain song starts to play in the background. It manages to describe everything she's feeling about Peter and her future all at once. Initially, Peter is upset with the song choice that Lara Jean settles on. He wasn't present for the moment, making him feel excluded, but once he gives the song a careful listen, he realizes that Lara Jean was right all along.
When Lara Jean first hears "Beginning Middle End" at the party, it's being performed by The Greeting Committee. Later, when she and Peter revisit the song, they listen to the version by Leah Nobel (who wrote the song). It's a song about an epic love that can weather anything, including the uncertainties that the future holds. It's about finding love that feels like it will last a lifetime. The lyrics align perfectly with Peter and Lara Jean's story across all three To All the Boys movies.
The "Beginning Middle End" lyrics feel like a description of Lara Jean's journey over the course of the movies. In the first verse, the narrator discusses finding love - something they've always wanted, but the search left them jaded. But then, when the right person came along, it was like a symphony erupted. Lara Jean is a person who loves love, which is clearly demonstrated by her love letters. She was deeply in love with her neighbor Josh for as long as she could remember, but she was always too scared to go for it. But when Peter came along, it was unexpected and perfect all at once.
Once they began dating, Peter truly became Lara Jean's beginning, middle, and end. The song even includes a line about being unsure of what the future holds, which is clearly reflected in To All the Boys 3's storyline about Peter and Lara Jean choosing colleges on opposite coasts. But despite of those uncertainties, the song describes being with that person for years to come. That's the future that To All The Boys: Always and Forever sets up for Lara Jean and Peter, making "Beginning Middle End" the perfect song for them.