Why am I always asked to enter my master password on certain actions when I'm already logged in?
You have enabled this as a security feature.
Master Password reprompt is a global setting that controls if the Master Password must be entered when performing tasks such as logging into a site, editing a site, viewing passwords, editing a form fill profile, etc. Once you check an action for which you wish to be reprompted, the setting will apply to every site, secure note, or form fill profile stored in your Vault. You can enable/disable this option from your LastPass Vault > Account Settings > General > Show Advanced Settings > Alerts.
Specific Entry/Note Feature:
If you only want to require a Master Password reprompt for a specific site or set of sites, use the ‘Require Password Reprompt’ option when editing a specific site in your Vault or via your icon’s dropdown menu (it is found under Advanced Settings within the edit window). LastPass is set to the ‘Normal’ security level by default, which only checks ‘Prompt for Master Password’ when switching or editing Identities. Please note that it is recommended you protect your accounts by logging out of LastPass, as that is stronger than this setting.