What causes page faults?

Question

According to Wikipedia:

A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual add

Answer 1

Resource allocation is a delicate balance between keeping primary storage available for use and preventing needing to go to secondary as much as possible. If a process tries to allocate memory and can't that's usually an exception and sometimes a fatal exception.

Essentially, you can't keep everything in RAM with no free resources available because when a program starts or asks for more it will crash.