In general, when cats lick, they are seeking comfort. According to the ASPCA, many cats who habitually lick are believed to have been taken from their mothers too early.
Kittens engage in both licking and sucking behaviors as they interact with their mothers, and some adult cats continue these actions if, as kittens, they were stopped too soon. Other cats use licking and sucking to comfort themselves when they are feeling stressed, worried, sick or bored. Some felines probably lick and suck simply because they find the behaviors enjoyable. Nothing needs to be done about the actions unless they appear to be compulsive, such as if the cats cannot be distracted from licking.