Is suffering always present?

I'm reading Thich Nhat Hanh's "The heart of the Buddhist teaching". When discussing the three dharma seals he talks about misconceptions of suffering. He says that teachings where suffering is considered a basic element of reality are wrong: > But sometimes only Two Dharma Seals are taught - suffering and > nirvana. .... But suffering is not a basic element of existence. It is > a feeling. When we insist on something that is impermanent and without > self being permanent and having a self, we suffer. The Buddha taught > that when suffering is present, we have to identify it and take the > necessary steps to transform it. He did not teach that suffering is > always present. This makes sense to me intuitively How do the different schools, i.e. TNH as well as those opposing, justify their position? What do they use as their reference? I have read this Q/A here however I am more interested in the references and how the different ideas are defended. The Q/A linked seems to be more opinion based, which is fine of course.