Avoid asking password if `su -m user command` is used under `user`?

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http://unix.stackexchange.com – If I run the script containing su -m user service userservice start under root user, I do not need to enter the password. But when I run it under user user, it asks passwords all the time. That really bugs me. Is it supposed to ask passwords for security reasons? Is there any way I could avoid asking password if su -m user command is run under user? (HowTos)