The friendship's specific texture
Friends to lovers romance requires investing in the friendship with the same craft attention typically given to the romantic relationship: building specific shared history, specific patterns of interaction, specific things they know about each other that no one else knows. The reader who has been shown what makes this friendship particular — the specific way they communicate, the specific things they have been through together, the specific role each plays in the other's life — will feel the romantic development as something that has been building in this specific relationship rather than something that has simply happened between two people who happened to be friends. The friendship should feel irreplaceable rather than generic; it should be the kind of friendship the reader wishes they had.