Why does it hurt when I touch the head of my penis when foreskin retracted?!

The glans are sensitive because they have had little contact. It would be like pulling your nail off and touching the skin underneath. You need to slowly desensitise your penis by pulling it back regularly. Leaving the skin back for several minutes, gradually to an hour. Time will desensitise the glans. This is completely normal. Nothing to worry about.

The foreskin should pull back easily and without pain. If you're doing this and there is no dried fluid or other thing to cause stickiness, this is not right and you should see a doctor. Your penis should work properly and painlessly, so don't be ashamed or reluctant to seek medical help. It's probably not a serious condition but you want to be sure -- this is your penis we're talking about.