A lost friend is a dead sunflower.
Back in high school, I did not have many friends, except for this one friend I considered my best one. Time passed, and eventually, it felt like I was talking to a wall. I made a myriad of attempts to save our friendship, and this is mirrored in the film. The robot is endlessly trying to keep the sunflower alive, so much so that the robot takes the sunflower to the sun. That’s how I felt whenever I made any attempts to save the friendship. In the end, I had to let go.