Cats often display "kneading" behavior towards their owners by pushing their front paws alternately against them. This behavior originates in the process of feeding from their mother's teats as kittens. If this means that cats express the same instinct towards their owners as their mothers, it wouldn't surprise me if your kitten is suckling your shirt because he/she sees you as a sort of substitute mother. It is also significant to note that kittens do purr while suckling from their mother's teats. The fact that your kitten does this means that it loves you.
