It could be any number of things that he would not feel comfortable saying outright. Sometimes, there really is a lack of chemistry, but that term is sometimes also used to cover up a more uncomfortable or serious issue.
I happen to think that people sometimes break it off too quickly because they claim they don’t feel chemistry or emotionally attached. Well, I’m not surprised…It’s difficult to be emotionally attached after a few dates. Perhaps if they kept it up for longer, and began to feel more comfortable around each other, they would develop an attachment or “chemistry”.