This is a policy set by you, or more likely, your systems administrator.
You can check what it is by going to Control Panel > Administrative Tools > Local Security Policy
The option you need is "Enforce password history". This will be set to an none zero value so that you don't just alternate between a small number of passwords. If you are part of a domain where these are controlled centrally by group policy then these options will be disabled (greyed out).
One place I worked had a policy that the passwords had to be changed every 30 days and the history was set to 12.
The danger with this approach is that users simply choose passwords of the form:
[passwordroot][number] (e.g. Password1)
and then just increment the number each month.