The fact that you are asking "why" indicates that you don't understand the process. In short, we are talking about random mutations within individuals of a species; the vast majority of those mutations are meaningless. Very rarely there are mutations that provide an advantage for those individual to survive and reproduce more efficiently within there environment (i.e., giraffes with longer necks). If the "new" version of this animal is then isolated from the main group (and does not interbreed), it will eventually evolve enough differences to be considered a separate species. In other words, the ability of humans to be self-aware is almost certainly a byproduct of other changes that had an evolutionary advantage at the time, in a specific environment (it was an accident).