Neil Irwin in The Washington Post had a similar complaint, and argued that people's pessimism arose from the fact that the indicators they see on a regular basis, such as gas prices, have been alarming to them.
But I think Mark Thoma has the real answer:
Part of the problem is the presumption in the question, which has been around for several years now and is basically "why are people so gloomy when the economy is doing so well?" If you ask instead, "why are people so gloomy when the economy has all these problems, reduced economic security, stagnant real wages, rising health care costs, falling home values, rising college costs, rising food costs, loss of employer based retirement programs, rising energy costs, worries about the future, etc., etc.," there's really no mystery.
I think both the "economist" at the meeting and the Washington Post reporter have limited capacity to think about what it must be like to be the median income household now. The median income household has now had (at best) many years of stagnant living conditions and worsening economic security.