I think it may be down to my British reserve. Over here it's really not de rigueur to boast, sell yourself or big yourself up in any way. (It's most evident when watching programmes like The Biggest Loser or America's Next Top Model. In the US version the contestants always say "I'm the greatest, I'm going to win!" and in the British version they are more likely to say "It's really tough competition, I'm not sure I've got a shot at all.") We're a self-effacing bunch and tend to come over all embarrassed and tongue-tied whenever we do anything that's actually quite good. So it goes completely against the grain to say "I'm a great writer and my book is amazing, buy it!"
In fact I have a seriously hard time trying to persuade people to buy my books. The best I can manage is "It's not everyone's cup of tea but you might like it," and then apologising for the price. It doesn't help that I'm really embarrassed at the idea of anyone actually reading my books, and well aware that there are some pretty fabulous books out there which are much, much better than anything I could ever hope to produce.
Little wonder, then, that British authors (well, me) don't do well submitting to American agents and publishers. You don't get far when your query letter says "I've written this book, I'm not all that good at writing but I hope you'll think it's OK enough to represent/publish." My standard query letter was generally just factual about the book (hook, plot, target audience) and me (moderately successful author of twee middle-aged fiction for a niche market).
I don't know what changed, but earlier this month while preparing a fifty-eighth submission for my YA/Crossover sci-fi novel I decided that, actually, I really believed in this book. I even liked it. I might go so far, in my query letter, as to suggest that it was good. I threw all my natural humility to the winds and wrote to the biggest publisher in the world telling them that I was a great writer and this was the best thing I had ever written (oh, the shame!) and I really thought they should take a look at it. I sent it off with the required 1,500 word sample, and prepared for my fifty-eighth rejection.
The punchline: they replied to ask me for the full manuscript. Random House liked my "partial" (writer speak) enough to ask for the "full".
So maybe, just maybe, it's worth forgetting humility once in a while.and, if you read your own book and enjoy it, admitting that it might just be pretty darn OK. In the end, though, whether I think it's any good or not makes no difference whatsoever. It's what Random House think of it that matters.