Why did Bertrand Russell care so much about the meaning of the word "the"?

Bertrand Russell saught to be an intellectual of integrity. And as many of his substance knows that our every presupposition is a self serving lie. Only by tracing simple truths may we form conceptual palaces with tenable foundations.

Articulating these most primitive conjectures give us the very language and dispositions for comparing and calibrating what comes of communicating among minds.

Principles and relationships are why.

Because he was bored after he proved 1 + 1 = 2:

https://www.storyofmathematics.com/20th_russell.html/

https://en.wikipedia.org/wiki/Principia_Mathematica