A budding author can put a lot of time and effort into ensuring their first 3 chapters are pristine and worthy of submission to an agent / publisher. After receiving a request for the rest of the manuscript it can come as a surprise to then receive a rejection and an explanation may not be forthcoming.
It's a bit like going on a blind date - you could be pleasantly surprised by what you first see but then waste a few hours trying to get behind the facade before you realise that there's little substance.
Will your story fulfil the promise of those opening chapters? To the reader, your first book is going to be either a great first date, a pleasant way to pass a few hours or a huge yawn - you can't please everyone but, if the chemistry is right, who knows? It might lead to a long relationship between the reader and you.