Fans speed up when the processor or graphics chip gets hot. The noise is not the problem; it is the symptom. The real question is why the chip is hot enough to need that much airflow.
Dust is the most common answer. Over months, the cooling fins and fan blades collect a blanket of fibres that blocks airflow. The fan has to spin faster to move the same amount of air, which makes it louder without cooling as well.
Soft surfaces make it worse. A laptop on a bed, blanket or lap sinks into the fabric and covers the air intakes on the bottom. The chip heats up, the fan ramps up, and the cycle continues until the laptop is moved to a hard, flat surface.
Some programs also keep the processor busy constantly. A runaway tab, a stuck update or background indexing can push the chip to its limit, and the fan responds by running at full speed even though the room is cool.