Every time you open a workbook that contains VBA Subroutines and Functions Excel performs a security check.   Depending upon your Macro Security Level, Excel does the following:

Security Level VERY HIGH:  Only signed macros from trusted sources and stored in special locations will run.   All other signed and unsigned macros will be disabled.

Security Level: HIGH:   Excel enables all VBA signed from trusted sources.   It disables all unsigned VBA.   It opens the workbook, but with disabled macros, your workbook does not  run properly.

Security Level: MEDIUM:  Excel warns you that the workbook contains unsigned macros which, if it were a virus, might be harmfull.   It give you the choice to ENABLE or DISABLE the macros before it opens the workbook.   Excel will enable Signed Macros from trusted sources automatically

Security Level LOW:  Excel enables all macros in the workbooks.   If the macro is a virus, you will be infected. 

By creating your own Signing Certificate, you can sign your macros and tell your computer that you are a trusted provider.   When you open any of your workbooks, you will not need to see the Enable Macro Dialog Box.