about # in preprocessor

Because the preprocessor must know which are its commands.
Using a 'special character' like # prevents name collisions with the compiler.

It makes it easier for both humans and computers to recognize which lines are preprocessor directives if they all start that way (think about it, if you're going through looking for preprocessor commands, it will be a lot easier if every line you're looking for starts with #). I don't think there's a particular reason # was chosen, besides the fact that it isn't used anywhere else in the language (that I can think of). It might be related to the fact it's a comment in most Linux/Unix shell scripts.

In the C and C++ programming languages, an #include guard, sometimes called a macro guard, is a particular construct used to avoid the problem of double inclusion when dealing with the include directive. The addition of #include guards to a header file is one way to make that file idempotent.