Why is it that light bulbs never seem to fail randomly during operation? Instead, they almost always expire after being turned on one too many times. It's probably happened to all of us; we hit the switch on the wall and a light turns on only to immediately burn out due to a broken filament. This annoyance is made very rare by energy efficient long lasting light bulbs, but for now let's stick to incandescent light bulb failure.
The end of life events of light bulbs are easily explainable. The source of light in an incandescent light bulb, the tungsten filament, gets viciously hot while adequate voltage for illumination is flowing through out. The intense heat constantly and consistently degrades the filament of the light bulb, causing oxidation and rusting. Eventually the filament sustains too much damage and the next time the light switch is flipped, the light bulb briefly starts but then burns out. It simply can't handle the voltage overload sent each time the electricity is switched on to the lamp.
Incandescent light bulbs are basically on the road to failure from the first time they are switched on. Eventually the startup voltage overload becomes too much for the weakened filament and it breaks. That's one way an incandescent light bulb can fail; another is due to an uneven filament. As the light bulb is used, certain points along the tungsten coil evaporate and thin out the filament. This can also cause coils of the filament to get pushed together. Heat builds up significantly faster in the thin areas of a filament, or where coils are pressed together. This extra heat becomes too much and the filament breaks or even melts.
It's the initial stress on the tungsten filament that leads to an incandescent light bulb's eventual failure.