(even) the best of friends must part
People must inevitably be parted from one another by some means or another. I know it's hard saying goodbye, but even the best of friends must part at some point. I treasure you above all my other friends. But I have to do what's best for my family, and so the best of friends must part.
(Even) the best of friends must part.
Prov. Even very good friends cannot stay together forever. Child: I don't want Debby to move away. She's my best friend. Mother: Sometimes the best of friends must part, honey, even if they don't want to.