There was a time, only about 30 years or so ago, that the definition of mental illness was essentially a democratic one, i.e. the majority decided what a mental illness was or wasn’t, based on what was defined as “normal”, or what the majority of people did in a given culture. So, if you were gay, tough luck! Since gays represent about 10% of the population, the other 90% could decide that their sexual orientation was either illegal (it still is in many countries) or an “illness” that required treatment.
With the emergence of the Women’s Movement, the fight for gay rights, the rise of multiculturalism, the decline of established religions, etc., a new psychological ethos began to emerge: An essential feature for the classification of a “disorder” was that it had to cause distress to the individual or to those around him/her. So for example, if you wanted to howl at the full moon all night long, that was OK so long as it didn’t bother you or kept your neighbors from getting a night’s sleep. And if it did keep them from a night’s sleep, you were more likely to be charged with a disturbing-the-piece misdemeanor rather than be carted off to the mental hospital.
Overall, this was a pretty significant shift in the diagnostic gestalt. It meant that we all had to learn to be more tolerant of each others quirks. Mental hospitals no longer were prisons for those who simply didn’t fit the norm. On the other hand, it has also enabled a lot of freaky behavior that simply continues to escalate until it turns destructive. One need only think about recent mass murderers who were “on the radar” for years but no one could do anything because their behavior, while aberrant, wasn’t harming anyone.