pronouns - If it's "yourself", why isn't it "hisself"?

There seems to be a bit of inconsistency with "-self" pronouns. Some are possessive while others are objective/subjective:

Consider:

• Yourself


• Ourselves


• Itself(?)

vs:

• Himself


• Herself


• Themselves


• Itself(?)

The first set use the possessive pronoun; the second don't. Why?

Similarly, is "itself" "it"+"-self" or "its"+"-[s]elf"?