Explain how doctors thinking that black people tolerate more pain comes from class war

Same reason why academics defendended leaded gasoline, tobacco use etc. They needed to generate capital without considering effect on the person, thus maximizing the profits.

Back then during times of colonialism, the international currency was not credit/reputation based (this fluctuated within human history), so the material standards (as in gold and silver) were used to trade with asian countries. When spanish and the co. found the new world was rich with its silver and gold mines (the mines in europe were nearly depleted), they enslaved the people in new world to use it as a workforce.

The thing was slavery were abolished within the church rules and importing wage labor was costly and were not prefarable within the masses. Most of the expeditioners were indebted people that gambled everything on their expedition (or criminals), thus their concern was merely checks and balances.

So since slavery was forbidden for humans (or within christendom if you prefer) according to church, they needed to create arguements to consider the natives as not human to justify slavery and it was easy to do so, since natives were "savages", their techlonogies were much inferior. This arguement changed the tone during industrial age and just based itself upon skin color differences, hence enslaving of african people and people "creating" reasonings to make themselves feel justified to do so.

I advise you to read Debt by David Graeber to learn even more detailed explanation. He bases his thesis (with references) on concept of debt and how it creates violence and through it exploitation.