Why do I need to add my Steam Guard code every time I open a game?

Steam Guard is an additional level of security that can be applied to your Steam account. The first level of security on your account is your login credentials, i.e., your Steam account name and password. With Steam Guard, a second level of security is applied to your account, making it harder for your Steam account to fall into the wrong hands.

When Steam Guard is enabled on your account, you'll need to provide a special code to log in from an unrecognized device. Depending on your Steam Guard settings, you'll either receive an e-mail with the special code, or you'll get it from the Steam Mobile app on your smartphone.

While logged into the Steam client, you can disable Steam Guard by clicking on 'Steam' in the top left-hand corner of the client. Then, select 'Settings', 'Security' tab and click 'Manage Steam Guard'. On the page that appears, disable 'Get Steam Guard codes by email”. After that, you will be required to re-login.