Moses said this to Israel right as he was about to die and they had to go forward without him, into a land full of enemies. In other words, he said it to people about to feel very alone.
Be strong and courageous, not because they were tough, but because of the reason he gives next: the Lord your God goes with you. The courage is borrowed from His presence, not summoned from their own nerve.
He will never leave you nor forsake you. It is doubled on purpose, never leave and never forsake, because your fear that everyone eventually abandons you needs to hear it twice.
So when the room is empty and it feels like people always leave, hear the promise God repeats: not this One. He goes with you, and He does not walk out.