The trouble is that smiling is easy to do. If flashing a smile can so easily convey good intent, it could be ‘hacked’ by unscrupulous individuals who want you to think that they’re trustworthy so they can exploit you. These kinds of ‘false smiles’ certainly happen in everyday life, yet we still generally trust smiles.
Smiling is generally an honest signal of someone’s intent and character because to do it convincingly means actually being engaged and attentive. That’s why smiling takes more effort than it first appears to – and why we trust people who do it.
Flashing a smile to take a selfie on your own might be low effort, but most smiles are part of active social interactions, and they need to be integrated into the broader context of the conversation.