Think about it: most people who have a pet will tell you how crazy the pet is. The dog does this stupid thing when the alarm clock goes off. The cat goes nuts when you give it an ice cube to play with. This is because we isolate domesticated animals from others of their type or breed when we take them into our home and care for them. Sure, they can be trained to know good from bad when it relates to us, the human owners, but removed from their peers, they have no objectively generated species-specific indicia to signal to them the metes and bounds of acceptable behavior for their kind, and this allows their own personalities to develop in a social vacuum. And go nuts.
The same thing happens to humans. We adapt to our circumstances, and our personalities veer away from socially acceptable behavior if we remove ourselves from interacting with a variety of different people on a regular basis, becoming shaped instead by the extent to which we can get away with following whatever natural impulses we have. As married folks grow older, they tolerate and accept behavior from each other that probably wouldn't be quite appropriate in public. Reclusive people are seen as eccentric or crazy.
This also explains why people who affiliate themselves with a larger group tend to go off the deep end a little, especially when the group is tolerant or even encouraging of a particular philosophy or ideology, along the lines of a "When in Rome..." mentality. Usually, the group impliedly or expressly sets up a new system of social rules, which the individuals adopt, internalize, and then try to extrapolate and apply these rules to the rest of society. Religious and political zealots, for example, completely at home in their church or among their fellow believers, are seen as stark raving lunatics when removed from this context.
This is why I'm reluctant to immerse myself in a crowd of vehement Kerry supporters at the rally outside my office building this afternoon. This is why the expression of eager conviction of people on downtown sidewalks carrying clipboards freak me out a bit. This is why the LaRouche Youth kids outside on the street are currently singing hymns, quite loudly, in the middle of downtown.