Honestly, reading so many people calling her "clueless" is really starting set me off. She's not clueless, she doesn't see being with him as an option, so she doesn't even entertain it as an idea.
She acknowledges his affections towards her, to a degree, but doesn't treat them as realistic because he is still a child in her eyes with a child-like attachment to the first person who gave him the time of day. There's a big difference betwixt being clueless and not taking someone's feelings seriously.