The Graduate of course became a very famous film, and often when this happens, people like me will say, "Don't bother going to see the film, there's so much more in the book." Well on this occasion let me say: if you've seen the film, don't bother reading the book. There isn't anything more in it. And the reason I can say that in confidence, without having seen the film, is that the book is essentially a script with speechmarks added and the tenses changed. It has no thoughts of characters, no interior monologue or anything underneath. Here there is no why.
Of course, as the book is about an unmotivated (or not clearly motivated) twenty-year-old college graduate, it is entirely proper that there is no insight into his behaviour. Why shouldn't we have to work it out for ourselves? Unfortunately, there are other authors who stay on the surface of things - Bret Easton Ellis, Cormac McCarthy - and who end up with works of great depth. The Graduate, probably because it displays little effort in engaging the reader, does not warrant the effort of thinking about it. (Perhaps part of the problem, too, is that the author was only 23 when he wrote it. How many great works are written in the author's early 20s? Incidentally I understand his recently published novel, New Cardiff, still uses this depthless technique, when he's surely old enough to know better.)
The Graduate is nominally a comedy, and it is amusing in places, but only briefly, and only as tiny oases in the middle of a vast desert of witless dialogue. The endless, unresolving two-handers predict the genius of Joseph Heller's Bob Slocum in Something Happened, with his pages and pages of futile disputes with his wife and children, but in Webb's clumsy hands are much more reminiscent of toe-curling music hall exchanges.
It ill behoves me, whose personal canon would include Money, A Handful of Dust, Something Happened and Brighton Rock, to complain about unsympathetic characters, but this too is a problem with The Graduate. When a book has little else going for it, at least characters you can root for would be something. But the protagonists of this book are like Tom and Daisy in The Great Gatsby (a book which will never again be mentioned in the same sentence as The Graduate): "careless people - they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they made..." and really, what one wants most of all is for them to get their comeuppance. Which they don't. Avoid.