The effect of a smile is powerful - even when it is unseen, Your “smile” comes through in your voice.
You must have a good time meeting people if you expect them to have a good time meeting you.
You don’t feel like smiling? Then what?
Force yourself to smile.
If you are alone, force yourself to whistle or hum a tune or sing. Act as if you were already happy, and that will tend to make you happy.
William James - “Action seems to follow feeling, but really action and feeling go together; and by regulating the action, which is under the more direct control of the will, we can indirectly regulate the feeling, which is not.”