Why many programming language are case sensitive?

Answer 1

Case-sensitivity greatly increases the number of tokens used to represent the language. As an example, escape sequences typically use an escape character (such as '\') followed by a single letter to denote a token. This would mean there'd be a maximum of just 26 escape sequence tokens. By using a mixture of upper and lower case letters, you double the number of available tokens.

In C++, the casing helps us distinguish between a built-in name (all lowercase), a user-defined name (a leading capital) and a macro symbol (all uppercase):

return (a built-in name)

Return (a user-defined name)

RETURN (a macro symbol)