Negative reviews are fine, but...

As the title says negative reviews are fine by me although, mainly out of necessity, I tend to only review my likes.
Why however, and it is very evident on amazon.co.uk , are reviewers who post a negative review of something almost always disinclined to suggest things we might like instead (and thus make a really positive contribution), while reviewers posting a positive review seem far more likely to suggest other things we might like as well?

Coming soon, reviews of:
Imogen Heap - Speak For Yourself
Thea Gilmore - Harpo's Ghost

and perhaps things you can't even imagine! If you don't like this idea then you should send me some suggestions...