They are in a hostile environment which they have no control. Passengers tend to be off center in the first place just being at the airport. They are expecting to board an aircraft and be whisked away in no time without one hitch. All this uncertainty and apprehension is just compounded when someone decides to delay the flight. There are many reasons why flights are delayed.
Some delays are for obvious reasons while others are for a more complicated reason.
By far the biggest contributing factor to a delay is weather. Weather is very disruptive to aviation and causes many problems for the traveler. The confusing factor to passengers about weather is that it is not necessarily the weather at their departure airport or the weather at their destination. Weather that your aircraft has to fly through to get to its destination. This all seems relatively simple but there is another factor to consider.