I'm referring to the surface of the glans and inner lining of the foreskin, which aren't really skin. They're a thinner kind of tissue known as mucous membrane, different in texture, and normally not dry like skin. I don't know what oils or whatever comprise the wetness under the foreskin, but a bit of soap does dry it up temporarily (and might sting, depending on the kind of soap).
z726 has it spot on, i'm also uncircumcised and began to retract my foreskin during puberty. In the shower is a good place as the hot water aides in making the foreskin softer and therefore easier to stretch. keep doing this over a period of time and it will eventually retract. be careful not to do too much too soon or it may become uncomfortable and possibly bleed from trying to pull all at once. :)
its bad you will feel itchy after using the soap
is the drying of the mucus membranes good or bad?
Just use water if you have a need to rinse under the skin. Soaps will dry out the mucous membranes under the foreskin, and who knows what various creams and lotions would do.
If you feel the need to make it looser, carefully stretching the skin a few times a day over the course of a few weeks will encourage it to expand so that the opening is wide enough for the foreskin to retract fully.