Paige McHugh and Luke Sandford have both suffered loss, making them wary of trusting again. Although they have developed a comfortable working relationship as jockey and trainer, Paige knows they have no business to be falling in love, particularly as Luke still has a fiancee who isn't quite his ex.
Severity reflects intensity, not value — “central theme” means a warning is a core part of the book, not that the book is bad.