Why do so many people have really crap relationships with their parents?

Why do you think it is? Is it because of how the parents brought them up? Is it because the way they think is different? Or that they were too controlling? A mixture of all this?
Can you think that this relationship can be fixed? If yes then how? How do you communicate with such parents, parents who they themselves don’t want to communicate?
And if you’re one of the people who are lucky to have a good relationship with your parent(s), why do you think you do so? What makes the relationship valuable to you?