This has been happening to me after restarts since upgrading from OS X El Capitan (10.11) to macOS Sierra (10.12). The ssh-add
solution worked temporarily but would not persist across another restart.
The permanent solution was to edit (or create) ~/.ssh/config
and enable the UseKeychain
option.
Host *
UseKeychain yes
Related: macOS keeps asking my ssh passphrase since I updated to Sierra