Naivety, gullibility, being to desperate to not be seen as 'uncool', being easily persuaded, and not feeling able to just say, 'No.'
They fail to realise that once a photo has been sent, it's likely to end up on the internet and being seen by millions of people.
And once it's there, it's there for ever.
They also don't seem to realise that if anyone sends a photo of a child, even if it's of themselves, it's illegal. It's child pornography. They can go to jail for it as a sex offender.