DIAGNOSIS The condenser coils are dirty or the unit is in a poorly ventilated space. The fridge is working overtime to shed heat.
FIX Unplug the fridge. Pull it away from the wall. The condenser coils are on the back. Use a vacuum cleaner with a brush attachment to remove all dust and pet hair from the coils. Ensure there is at least 3 inches of clearance on all sides, especially the back and top.
IF THAT DIDN'T WORK The door gasket may be torn or not sealing. Close the door on a dollar bill. If it pulls out with little resistance, the seal is bad. Clean the gasket with warm soapy water. If it's cracked, it must be replaced.
HEADS UP The sides being warm is normal during a cooling cycle, but they shouldn't be excessively hot. Continuous operation in a hot room can permanently shorten the compressor's life.