You must change your password in order to ensure the security of your computer account. If your password were not changed on a regular basis, your familiarity with your password would eventually lead to it's compromise.
Once another person has acquired your password, they can use your computer account. They can even use your computer account to attack other machines. If they use your account to attack a machine, it will look like YOU attacked that machine and the authorities will come looking for YOU!
Why can't I use a variant of my old password?
If you have ever watched a detective show on TV, you know that people are compromised by their habits. Someone can observe your daily habits and predict what you will do next. The same is true for your password choices. You get into a habit of using a password and it becomes easy. Once someone learns your password, if you have used a variant of that password, it is not going to take much more than a few minutes for that person to guess your new password and break into your computer account. And then they can use your account to commit crimes in YOUR name!
Why must I use multiple character classes in my password?
This requirement was instituted to help you create a better, more secure password. There are only 26 letters in the English alphabet. However, adding the ten numbers and the twenty to twenty-five non-alphanumeric characters, you have over sixty characters to use in creating your password. Your password is also case sensitive, so you can use upper and lower case letters too. This increases the security of your password tenfold and decreases the possibility of someone guessing it.