The weave of some types of natural fibers, such as merino wool, is tighter. This means that they don’t irritate the skin nearly as much. When the overlap is greater, it causes the fibers to rub against the skin with more force, which can cause irritation. There’s a simple solution that involves a plastic bag, and a household appliance.
Put the irritating sweater in a bag of plastic and place it into the freezer. Set a clock and remove the sweater about three to four hrs later. Place the garment in an area that is shaded (NOT the sun) and wait for it to defrost. The result should be a sweater that is softer and has a tighter weave. This is a good hack to use on sweaters that have lost their shape or stretched over time.