For the following I would like to note that the meaning of life is to create more of; and better versions of yourself.
For most of history Darwinism worked fine and helped humanity by shedding the weak and the lame. However modern society has advanced so far that Darwinism isn't as applicable. Government hand outs and diversity hiring have played a huge role in artificially inflating the affluence and influence of groups that would otherwise have died off.
Stereotypes exist for a reason. Anyone who rejects that notion is either uneducated on the matter or willfully denying reality due to bias. Not to say that all stereotypes are accurate, but they do exist because some are quite accurate. Some are based on the culture of a group, a genetic disposition a group has to a specific attribute, or long running rumored ones (like before widespread pornography some folks actually believed Asian women had horizontal vaginas, no I am not joking).
A homogeneous society is usually a happier and safer one. In most places that are lacking "diversity" the governing and social systems work better. Whether is is due to a hierarchy among the races and ethnicities of the world or some sort of internal social mechanism deep within the basic functions of all hominids, I can not say for sure, but I can say for sure that when a society is homogeneous it works better and for longer. There is far more to a working society than ethnicity and race, obviously. Religion, political leanings, even food choices can, have, and will continue to contribute to rifts in a society.
Skin color isn't the end all be all of race, it's just a very clear marker thereof as opposed to the sometimes subtle racial differences between less obvious races like the different European races; for example, a Scot or Irish is more likely to be ginger than a Frenchie, so if you see a ginger you're more likely to believe they're a Scot or Irish than a Frenchie. But these are just superficial things, for the most part. What really counts is what's in their genetics.
Genes are a very peculiar thing. Most folks don't really understand them or how they work, but put simply they make us who and what we are and determine literally everything about us. Our height, our intelligence, our hair density, our musculature, literally our everything. The Dutch are some of the tallest people in the world, as such it is fair to say, Dutch folk posess a "tall" gene, meaning throughout the history of Holland, enough tall people made babies to make being a tall a notable Dutch trait, this is what is known as a genetic trait. Genetic traits can be "dominant" or "recessive"; but with genes, nothing is certain, hence the non-definitive terminology. A recessive gene can pop up out of nowhere after lying dormant within a bloodline for generations and in doing so could give a person green eyes instead of the brown which both parents had. For example, if a mostly white person's genetic makeup is 2% congolese, there is a chance; however small, that the person could procreate with someone who is 100% 1488 Nazi megaracist white, that their child could have the tightly curled hair common among afrikans.
Certain racial and ethnic groups hold certain genetic traits, but are not always exclusive to those groups by way of how randomly genes work. However, when you mix genetically similar peoples you are more likely to get the traits inherent to whatever they may be. For example, two blondes have a higher likelihood of having a blonde child because they share the gene for blonde hair. Two smart people are more likely to have a smart child because they possess the genes that contribute to being smart. Mutations can occur, giving us super geniuses, or things like The Hartley Hooligans though.
TL;DR
Mixing races is bad because some have better traits than others.