Option Explicit can be put  in automatically by Excel (for new modules).
It's in Tools/Options/Require variable  Declaration.

Never trust global variables  to retain  a value between separate "runs" of
the code while the spreadsheet is open. On occasions, Excel will "reset
your project". Sometimes it warns you and other times it doesn't.

If you want a value to have some permanence, you're better off putting the
value into a cell  - possibly on a hidden sheet.

I will also tend to put calls to an "Initialise" routine into every
subroutine on occasions. The call  checks an "Initialised" Boolean
variable - if that is false, it does the initialisation then sets it to
true.