He really loves Nodame, no one else would have put up with her "craziness"; Kuroki had a crush on her, until he realised just how "strange" Nodame was, and that soon ended.
On the other hand, Nodame was the kind that proclaimed her love for him all the time, but she seemed to be on her own and doesn't really need Chiaki once more, which I guess make Chiaki really hesitates. He doesn't know if she really does like him, or is just superficial. Yet I believe Nodame, in her own way really likes Chiaki, but the two are so different that they always misinterpreted each other's actions. LOL. how cute.
Put it simply, Chiaki is kinda shy with relationships, and he probably felt really horrid if he thinks Nodame is fine without him. See all his jealousy rising at various events? ^o^