When you open the Minecraft launcher for the first time, it asks for email/password to log in. Unless you manually log out, clear the cache, or have another profile log in, that's the only time you should be asked to manually log in.

However, every time the Minecraft Launcher updates, you will need to log back in as either it does not automatically log in or it forces a sign out (as if you clicked the logout button). This does not happen if no update is available.

To reproduce:

1. Open the Minecraft launcher.
2. Log in.
3. Close the Minecraft launcher, don't log out.
4. Open the Minecraft launcher.
5. Have it check for updates and update accordingly.
6. You will be logged out of your profile inside the launcher.