XOXO means "hugs and kisses" but why?

The earliest mention of X and O together dates back to the 1960s. The practice of writing X goes all the way back to Middle Ages, a time when few people were literate. People (mostly illiterate) in those ages used to sign documents using the letter X, probably because it was simple and had religious connotations (Christian cross). It was associated with 'Christ' long before that (the Wikipedia article on the letter X says that X is sometimes used as an abbreviation for 'Christ', so 'Xmas' means 'Christmas').

So by signing X, you're saying In Christ's name, I assert___. Some sources I checked say that the signee would then kiss the X to demonstrate that what was written in the document was true. This can also be confirmed at the Wikipedia article on 'Hugs and kisses'.

According to another article on Today I found, “X” first started being used as a substitute for “Christ” by religious scholars about a millennia ago, which is actually how we ultimately got Xmas as an alternative name for Christmas.

The use of X to indicate a kiss is attested since 1763. The use of O to mean a hug is believed to have arisen in North America. Etymology Nerd and Mental Floss say that it was popularised by Jewish immigrants in the United States who refused to use the cross. That article goes on to say that 'It could've been formed as a contrast to the x, or adopted because of an aesthetic similarity to what a hug looks like'.

Some other sources such as Today I Found Out also suggest that it got its meaning from resembling the arms encircling someone in a hug. Today I Found goes into great detail on the topic and says that the Jewish immigrants (illiterate) refused to use the Christian cross and used O instead to sign their letters and other documents. An article on Mashable.com adds that the letter X then transformed from a religious one to a common romantic one.

Marcel Danesi, a professor of linguistic anthropology and semiotics at the University of Toronto, in an interview with Mashable says:

"The X has always been a Christian symbol, and it is the first Greek letter in the name of 'Christ,' Danesi says. "As far as I can tell, official letters in the medieval period and even after were literally sealed with the X — sealed with a kiss of faith, I guess."

Danesi also explains that illiterate people used the X to sign documents. It was customary for them to plant a physical kiss on the X.

From Washington Post:

And in a discussion chain on the American Dialect Society, linguist Ben Zimmer, in a search of newspaper archives, found “xoxo” and “xoxoxo” used in personal ads from about 1972.

The reason why hugs and kisses is rendered XOXO and not OXOX is unknown. Maybe it because of the influence of Tic-tac-toe. However, according to ScoopWhoop, it's under debate.