Songfacts®:

• This is the opening track of Blood Harmony, the debut EP by Los Angeles singer-songwriter Finneas. The record marks Finneas' first solo release after concentrating on co-writing and producing material for his sister, Billie Eilish. The title refers to the vocal chemistry of siblings who sing together.
• This song features autobiographical lyrics about Finneas' childhood best friend. Their friendship lasted throughout high school before falling apart.
  
  I lost my mind, and nobody believes me
  Say, "I know that he don't need me
  'Cause he made a little too much money to be twenty and sad"
  
  Finneas explained to Genius that while he was experiencing "wonderful and amazing" success with his sister, he was unhappy as this friendship had fallen apart. He added: "That person and probably other friends of mine were like what is he so bummed out about, his life is great."
• Finneas admitted to Coup De Main that he was nervous releasing "Lost A Friend" as their relationship had picked up again. He knew his buddy would hear the song, so he sent it to him first, saying, "This is how I was feeling at the time, and I'm glad to have you back in my life."
• Finneas told Radio.com this contains a lyric he's especially proud of:
  
  I know I'll be alright, but I'm not tonight
  
  He explained: "There's a lot of music or art sort of written about this forever lonely, forever sad thing To me in my life, it's like 'ooh, I'm really intensely unhappy right now, and I'm sure I won't be in two months. But right now I'm bummed.'"