Credit score

When the bank calls and alerts me to the fact that someone else has used my credit card, I tell them O.K. then void the credit card, and send me a new one which they did quickly.  Now that new card makes my credit score go down.  I can't see why my credit worthiness is lower.  As a matter of fact, I have a rather high net worth and I am living way beneath my means so that I can leave enough for my kids retirements, new, houses, or their children's education.  I was also dinged because I signed a co-borrower mortgage for my son's fiancee's new house.  If she should fall seriously ill or whatever, I will just pay off the mortgage and have them pay whatever rent to me that they can afford.  I don't think all of this merits a mediocre credit score, but it really doesn't matter because I can not conceive of any reason that I will have to borrow money for anything during the rest of my life.