Speaking to Husain as part of the show, Mr Thunberg said his daughter had struggled with depression for "three or four years" before she began her school strike.
"She stopped talking... she stopped going to school," he said.
He added that it was the "ultimate nightmare for a parent" when Greta began refusing to eat.
To help her get better, Mr Thunberg spent more time with Greta and her younger sister, Beata, at their home in Sweden. Greta's mother, opera singer and former Eurovision Song Contest participant Malena Ernman, cancelled contracts so the whole family could be together.
The family also sought help from doctors, Mr Thunberg said. Greta was diagnosed with Asperger's - a form of autism - aged 12, something she has said allows her to "see things from outside the box".
Image copyrightKENA BETANCUR/GETTY IMAGESOver the next few years they began discussing and researching climate change, with Greta becoming increasingly passionate about tackling the issue.
As "very active" human rights advocates, Greta accused her parents of being "huge hypocrites", Mr Thunberg said.
"Greta said: 'Whose human rights are you standing up for?', since we were not taking this climate issue seriously," he explained.
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Unhealthy unsound underdeveloped. Sad little girl 16 going on 8.
