Why is the oil filter so hard to loosen and tighten?

An oil filter should be tighten only by hand, never with a wrench, this is a very common mistake.

You Must allways fit a brand new rubber gasket, because it "Grows" once in contact with the oil : sealing+tightening the oil filter. You can check yourself comparing thickness of a new one to that of a used one.

Remember : the oil filter rubber gasket -unlike others- swells in thickness, so help yourself and do not overdo it!