Why does SQL Server resume mirroring after server is restarted?
Is that normal?


This is the default behavior to automatically resume mirroring when the mirror comes up (provided no errors encountered). It will be in suspended state if there is a REDO error. 


  I want to resume it manually after my verification... 


If it resumes by itself then everything is good. You dont have to worry. If it does not then there is a problem and you will start troubleshooting.