What caused the final separation between star girl and Leo?

The final separation between Stargirl and Leo was primarily caused by Leo's struggle to fit in with his peers and the pressure he felt to conform. After Stargirl's unique and nonconformist nature drew both admiration and ridicule, Leo chose to distance himself from her to avoid social backlash. This decision ultimately led to a heartbreaking moment where Stargirl realized she couldn't change who she was for Leo or anyone else, resulting in their emotional parting.