Is there a moment?

I often wonder if there's a particular moment, a specific second or millisecond when someone makes a conscious decision to change their friendship. Or is it just a gradual lack of effort that eventually leads to the simple errosion of the ties that bind?

It often starts the same; an email isn't responded to, then a second. Sometimes there's a reply several months after the first email. The next reply takes even longer, and suddenly, by the time the person realizes it's been as many months as it has, it becomes a question of whether to bother recapping everything that's transpired or to just gloss over things with a general response. When does it change to the point that there's no repair and the one-sided effort is just too much? And at what point do people become so emmotionally bruised that they don't want to open up again? And at what point does a person just have to accept that the general essense of the friendship has changed and will never be the same again?