Why am I able to switch user only using hyphen?

If I try to switch the user using sudo su - kshitiz it works, and doesn't ask me for any password.

But if I do sudo su kshitiz it asks me for that user's password.

Why is this? I understand that hyphen loads that user's environment variables where's no hyphen keeps my own.

So then why does it ask me for target user's password when I want to switch while keeping my own variables, but not when I switch and load target's profile?