The more expensive the wool garment, the finer the structure of the wool fibres in terms of form, i.e. the better the degree of softness and curl. The disadvantage is that the fibres are more likely to tangle and pucker.
This is the main reason why wool sweaters pucker. Pilling can also be caused by physical friction in everyday life.
For example, pilling is more likely to occur in pockets, cuffs and chest areas where wool is often rubbed by foreign objects or worn.
When spinning wool, the manufacturers relax the twist of the yarn to make it feel soft, which causes the fibres to hold together more loosely.