…mental illness?
I have to say, Clayton Cramer dug up quite a gem here:

It is proposed that happiness be classified as a psychiatric disorder and be included in future editions of the major diagnostic manuals under the new name: major affective disorder, pleasant type. In a review of the relevant literature it is shown that happiness is statistically abnormal, consists of a discrete cluster of symptoms, is associated with a range of cognitive abnormalities, and probably reflects the abnormal functioning of the central nervous system. One possible objection to this proposal remains–that happiness is not negatively valued. However, this objection is dismissed as scientifically irrelevant. [J Med Ethics 1992 Jun;18(2):94-8] [emphasis added]

If we add the disease of “happiness” to that of depression, anxiety, OCD, and all the other mental diseases from which 20% of us are said to be suffering and could be medicated, we approach a mental illness saturation point.
All of us are mentally ill.
And each of us commits three felonies a day. Which means we are all criminally insane in the eyes of our rulers.
So be happy in your pursuits!
UPDATE: I was sent the text of Dr. Bentall’s proposal to classify happiness as a mental disorder by an emailer who argued that it was parody, and who said:

When something looks too silly to be true, it sometimes is, so it pays to look at it a bit more closely.

I just did, and I find it a relief that he didn’t himself maintain happiness is diseased.
However, I don’t think Bentall was being silly; he was making a very serious point.
Looking closely at the text, it is obvious the author (himself a forensic clinical psychologist) was writing in protest of his profession’s dominant narratives, and meant to ridicule them. Excerpt:

once the debilitating consequences of happiness become widely recognised it is likely that psychiatrists will begin to devise treatments for the condition and we can expect the emergence of happiness clinics and anti-happiness medications in the not too distant future.
The second, related objection to the proposal that happiness be regarded as a psychiatric disorder points to the fact that happiness is not normally negatively valued. Indeed, it is testimony to the insidious effects of happiness on some of the greatest minds in history that some philosophers have argued that the pursuit of happiness is the ultimate aim of all human endeavours. However, it is notable that even some of those who have been rash enough to advocate the greatest happiness for the greatest number have been explicit in rejecting those extreme forms of happiness associated with gluttony of the senses (32). More importantly, the argument that happiness be excluded from future classifications of mental disorder merely on the grounds that it is not negatively valued carries the implication that value judgements should determine our approach to psychiatric classification. Such a suggestion is clearly inimical to the spirit of psychopathology considered as a natural science. Indeed, only a psychopathology that openly declares the relevance of values to classification could persist in excluding happiness from the psychiatric disorders.

Whether Dr. Bentall truly wanted happiness to be added to psychiatric disorders is not the point. He is arguing logically that by using his own colleagues’ criteria, there is no reason not to consider happiness a form of mental illness, and I think he is right in that respect. It is a paradox.
Moreover, Bentall (whose happiness paper generated much anger) is on record as saying that there is no clear line between the sick and the well:

In Bentall’s view, there is no clear boundary between the sick and the well. While this feels plausible for conditions such as anxiety and depression, the suggestion that psychotic experiences run through the general population on a continuum from bland normality to florid schizophrenia has always met with resistance from psychiatrists. The received view is that psychotic symptoms, such as hearing voices or believing that thoughts are being inserted into one’s brain by alien forces, are by definition beyond the realm of ordinary experience.
Not so, says Bentall. In fact, otherwise well- adjusted individuals commonly report “schizotypal” experiences such as these. He cites a study of the “normal” population of Dunedin, New Zealand: “20.1% of the sample were recorded as having delusions . . . 12.6% were judged to be paranoid.”

It makes about as much sense to say that happiness is mental illness as to say that sadness is mental illness.
To take sadness further, if what they call “depression” can be a normal reaction to depressing events, then why aren’t people who are happy in the face of depressing events even more abnormal than those who find themselves depressed?
Is the rule along the lines of happy = good, and sad = bad?
It’s obvious that Bentall meant to issue a challenge to his profession, and in that respect, I think his paper is quite serious.
At no point does he suggest that he wants to treat happiness as disease. Only that if his colleagues should — if they practiced what they preached.
And if they do as he suggests and we reach the saturation point, mental illness will have become the norm.