No excuses necessary.

If someone wants to hear "on your left" they're free to say it to themselves. In normal riding with room to pass safely, no one has any obligation to speak, wave, or give the look. And I suppose that if it makes someone happy to discourteously complain that a stranger was discourteous by not talking, they're free to do that too. For myself, I'd prefer that you say something nice or say nothing at all, but either way I'll just keep riding.