How much should an adaptation change things?

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This morning, I saw A Clockwork Orange for the first time. As usual after I see a movie, I check out it's Wikipedia page. According to wikipedia, a few changes were made, such as the character of Mr. Alexander.

A few of the differences include:

Critic Randy Rasmussen has argued that the government in the film is in a considerable shambles and in a state of desperation while the government in the novel is quite strong and self-confident. The former reflects Kubrick's preoccupation with the theme of acts of self-interest masked as simply following procedure.[37]
One example of this would be differences in the portrayal of P.R. Deltoid, Alex's "post-corrective advisor". In the novel, P.R. Deltoid appears to have some moral authority (although not enough to prevent Alex from lying to him or engaging in crime despite his protestations). In the film, Deltoid is slightly sadistic and seems to have a sexual interest in Alex, interviewing him in his parents' bedroom and smacking him in the crotch.
In the novel, it is completely clear that Mrs. Alexander died of injuries sustained during the gang-rape. Kubrick's film has Mr. Alexander rant that his wife died a few months later during a flu epidemic, though he still blames her death on the rape. He calls her a "victim of the modern age".
When Alex re-encounters Mr. Alexander in the novel, Burgess portrays him as a basically decent man struggling to maintain his sanity after his life has been ripped apart. In the film, Kubrick turns Mr. Alexander to a less mentally stable, very traumatized and angry figure whose hair has been teased out to give him a faint resemblance to Beethoven.

Now, I wonder why these changes were made. And why they had to be made to the script. Is it okay for a director to change a story as he or she sees fit so it will tell the message they see, or should they aspire to be as faithful to the source material as possible? If so, or if not, why?

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In this case? The movie was simply made to be darker and more cynical than the philosophical bent of the original novel. Rather than portray Alex's crimes as something he outgrows in order to become an adult, as Burgess intended, it simply glorifies in them and in Alex's eventual humiliation. It doesn't have much of a point other than to effective in depicting those two things.

I suppose in that case, you could call it a very bad adaptation, albeit an extremely good movie. To my mind, a good adaptation will try to convey the gist and ideas behind it's source. Mutilate, fold, spindle, but remain faithful to the idea behind what you're mutilating. If the characters are the same people, and the overall journey they undergo is similar, then the exact forms are not important.

Straighforward from-the-page-to-the-lens adaptations? For the birds. It turns out different mediums aren't as effective with the exact same material, and what looked good in a book can be terrible and plodding on-screen. For a wealth of information on this, see Potter, H.