Felix Zuck revealed a secret to Yusuf Zuck, a secret that Sara was not his biological daughter; he had never touched a woman in his life, including Yusuf's mother—those years had merely been about sharing a household together.
His many years of indulging in alcohol and reckless gambling were, in fact, forms of self-destruction, because he didn't know what else he could do with his life aside from these two things.
Yusuf initially thought he was joking, but as Felix spoke with solemnity and seriousness, Yusuf stopped smiling, and he felt like the hospital room was suffocating him.
Indeed, after hearing Felix's account, Yusuf felt breathless. He felt the futility and sadness of Felix's life, and he was also feeling lost about the family they had formed together.
Sara wasn't actually Felix's biological daughter either; she was a child he had found, and aside from Felix and his deceased mother, no third person knew about this secret.