You might pretend not to care because you have somehow learned the faulty ideology that someone who does not care holds all the power. Another reason that people pretend to not care is because some experience has taught them that caring about things leads to more possibility of getting hurt. Lastly, you might also pretend not to care because many people think that caring about things makes you look too sensitive or desperate, which is why they may feel it is better to just be detached and cool.