You don't let emotions thwart your mission and lose out on important details and samples. Leon lost the LAS PLAGAS sample to Ada because of his affection for her. That's weak. Especially after she backstabbed him.
Its not like he's getting her. She seems like she moved on and is more concentrated on her goals. Something Leon should do (and seems is doing in RE6 hopefully).
"The man who smiles when things go wrong has thought of someone to blame it on". PSN: ElusionM