The fan will come on when the CPU gets too hot.
Or depending on the age of the laptop and the environment, the intake for the fan and / or the intake duct / cpu heat sink could be full of dust.
If it is dust, then, with the laptop off you can use a vacuum cleaner to try and suck the dust out. Or send it in to the Samsung repairer to get them to look at it or clean it.
I never let my laptop sit flat on a desk. I have a cheap plastic frame that angles the back of my laptop off the desk. That way the intake for the fan is not obstructed, hence keeping my i7 cooler.