>>534771375
Saiyan tails likely are packed with nerve endings, it's why grabbing one can make them go limp, it's overwhelming, a sensory overload. They can be trained to be less sensitive though, as seen with Goku, Vegeta, and Nappa. Biting and any rough activity can potentially hurt a lot, but some Saiyans might have some pain-pleasure crossover in the brain given their natural inclinations; stroking or pulling the tail (or even gentle biting) probably does feel very good though because it's a lot of gentle pressure applied to so many nerves, which can translate to arousal. Possibly even a fetish in some cases.