Why does the code change on my debit card when I get a new one?

Every time my debit card expires and I get a new one, why does the 3-digit security code on the back not the same as it was on the previous card? Is there a way to write a written request to the credit union that I do not want this number changed on my next debit card? Why do the cards expire?|||It's changed for security reasons - which is also why cards have an expiration date. You can't change that.