Right, I had a hair dryer in the 70's. They weren't much fun.
Most (all?) laptop fans are tied to a temperature sensor. When the sensor sends a signal saying it's too hot, the fan turns on. When the sensed temperature falls back to a predetermined point, the fan turns off. Some newer computers have multispeed (high, med., low) or variable speed fans.
Your problem could
be caused by an bad sensor, a failing fan, or something worse. The most common cause of these complaints is dust build-up, however. A can of air sprayed into the vents is the easiest. simplest fix to make.
Another thing that effects how often/long/hard a laptop fan runs is vents being blocked when placed on soft surfaces like bedding, sofa/chair cushions, carpets, etc.
If you have a manual (either paper or electronic copy) for your laptop, check the maintenance section for cleaning procedures. If you don't have one, either try a on online search for the manual using your manufacturer and model number as search criteria, or post the info here and I'll search for you.
You could try a program like SpeedFan - there are others, click here for SpeedFan, to monitor your internal temperatures. You might be able to correlate temperatures with fan activation.
Let us know what you find.