This is expected behaviour, you're just "signing" the message with your key at this stage. You need to import someone else's public key before you can send an encrypted message /to them/.
Thanks for clearing that up.
You need the public keys of your contacts before you can send them encrypted messages, so encryption is normally enabled on a per contact or per message basis. I think the signature is there to provide a level of assurance to your PGP enabled contacts.
I also can't use S/MIME even though I got a certificate, I don't know how to use it.
[1] https://gpgtools.org/