Why is this a pointer

Why is this a pointer?

Because it is, and it is just the way the programming language was designed. (I think it is also because it was left over from C, but I am not sure).

rather than a reference?

If you need a this reference, use *this.

Can it never be NULL?

...no. How could it be?

Why is "this" not a reference?

See Bjarne's answer here

Because "this" was introduced into C++ (really into C with Classes) before references were added. Also, I chose "this" to follow Simula usage, rather than the (later) Smalltalk use of "self".