A good analogy might be to ask why it feels so good to scratch the area on your back that you "can't get to".

Human skin tends to accumulate dead skin cells on the exterior, as they are being constantly replaced by fresh new skin cells. These dead skin cells will normally fall off in the air, but when covered by fingernails, a layer can accumulate that will prevent you from feeling the movement of air in that area, increasing the threshold for your level of sensory perception.

When sliding a fingernail under another, you are scraping off a layer of dead skin cells from the underside of the nail and from the area of skin normally protected. The heightened sensory perception allows you to feel the fresh movement of air and moisture over that skin, resulting in a feeling of pleasure that is at a higher level than usual.

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