What motivated imperialist domination and colonization in Asia? What justified colonialism and imperialism in the region?

So, what motivated imperialist domination? By that, you probably mean specifically European imperialist domination. It began apparently with the Crusades. Europeans soldiers wanted to access more directly the trade of Asian goods, coming from South Asia, East Asia and different parts of West Asia, which they found very useful and valuable. Most European traders could not reach the Levant on the Mediterranean because the Venetian militarily dominated this trade route. So the Iberians —often led by the Genoese— sought to go around Africa, though Columbus the Genoese found America instead. Then Vasco da Gama found another connection to Asia around the Cape of Good Hope. He reached what we now know as the state of Kerala and established the first European trading port there. Now, two important developments favoured the Europeans. One was the discovery of America, especially the silver mines of Potosi. The Mediterranean Europeans utilized that silver to buy goods from Asia —the Asians didn’t have much need for anything European in exchange, as far as goods are concerned. The other one was the distinctive advantage the Europeans had in engaging in open-sea navigation. This is something that Asians had not really been able to do. The Chinese Admiral Zheng He, around the beginning of the 15th century, did reach Africa, but he never made it beyond that —he largely engaged in coastal navigation. Besides being able to direct vessels upon the open sea, the Europeans also had a sort of navies —militarily equipped ships— with which they were able to engage in raids. So, in a way, they were like pirates. For more than two millennia, coastal trade in Asia was controlled by the sultans, rajas and mandarins of the regions where the ships docked. Thanks to their military advantage, the Portuguese, for example, were able to take over Goa, then Macau and then Manila, from the Spaniards. So, in this way, gradually, the Europeans penetrated this region.

Now, what justified imperialims? I think that for the Portuguese, the Spanish and even the Dutch there was no need for justification. They just wanted the spices, the gold, the silver and the slaves. By that time, they had already developed slave labour, which was also practiced by the Islamic societies in the Azores, the Canary Islands and other places. With the Portuguese and Spanish there was also the Christian idea that the world would come to an end and that before that happened the world had to be Christianized. In Asia, they were not as successful at that as they were in Latin America, but that was a justification. As for the Dutch, they were just interested in trade, in dominating as much as the could through trade. They were the ones who had no cover-up about their intentions. If they did, I don’t know what it was. By the late 19th century, the British and the French developed these ideas of Social Darwinism that they were inherently a superior people and therefore their undertaking was a civilizing mission. Also, once they became economically superior and after having destroyed much of the Asian industry, which was not a mechanized industry, they started talking about helping Asian countries grow economically because they had all this technology.