what do christians (and people in general) mean by "feeling empty inside"?

[Mister Agenda]

There's a concept called 'the hedonic treadmill' that suggests we tend to return to our natural level of happiness over time following a disturbance (whether the disturbance made you happier or sadder). So an optimist who lost a loved one would tend to return to being an optimist over time, though they would still suffer from grief and possibly depression in the short term. A gloomy person who won a hundred million dollar lottery would find that the money didn't make her happy, and an sunny person in the situation would find it didn't make them much happier than before over the long run.