For Axel, seeing Leah so broken and devastated is a punch to the gut. He promises himself he'll do anything to bring the light back into her eyes, no matter what it takes. He would be shocked to learn that Leah is in love with him.
Little by little, as he does everything in his power to bring Leah back to the sparkling, energetic beauty he's known, he falls in love with her, too, even though he feels that he's betraying Oliver's trust. Before he can tell him, Oliver finds out about Leah and Axel's relationship and is outraged. He insists Axel prove his love by letting her go so she can fully experience life at college.
Axel agrees to break both of their hearts, and stoically breaks up with Leah so she can start a new life, not realizing the repercussions that will affect them both for years to come.