Well, there are lots of 'good' reasons to delay a plane from flying.Most of the reasons are about the weather.When we have a thypoon,Strong winds and heavy rain make the visibility very low.So it is very dangerous to let a plane depart or arrive at this condition.
Sandstorms(usually in the arabian,sahara and south U.S.A) also make the visability drop very fast,so planes are not allowed to take off or land when the sand storm is striking.
Also bombs or terorrist attacks may delay the plane too!When the departure countrysuspects there is a bomb or terrorists hiding,they will delay the plane andf do a searchg to ensure passengrer's safety.
Mechanics might also delay a plane.If the plane has something broken or missing,it must be repaired before it could fly again.This due to the international rules of aviation safety.If the damage is found just before boarding,the plane will probably be delayed.!!