It's the power of the mind. For example: (I'm going to use names to make it less confusing.) Ok, Lets say that Sally put a curse on Mike. Sally put a lot of effort into her curse and she truly believes that the curse will work.
I believe that all of Sally's efforts and beliefs play a good role in her curse. Thus, she is using the power of her mind to empower the curse. I believe that Sally's efforts and beliefs can actually help make her curse manifest into reality. Also, if Sally decides to tell other people, or Mike himself, about the curse, she's using manipulation to empower her curse as well.
Once Mike finds out that Sally has cursed him, he will get paranoid and bad things will start to happen to him. At fist he will notice little bad things starting to happen. For example, He loses his keys. He would probably shrug it off once he finds them and not really think about the curse.
Then, another day, he goes for a walk and steps in dog poop. He probably wont think much about the curse at this point but he does notice that a bad thing has happened again.
Then, the next day, his alarm doesn't go off and he is late for work. He will most likely shrug this off too but the curse may be in the back of his mind.
Then, the next day, he accidentally slips and falls in a puddle. He notices that a bad thing has happened yet again and he starts to wonder about the curse.
Then he comes home one day to find his old dog dead. This starts to make him paranoid and he thinks about the curse more and more. He now is starting to believe in the curse.
And so on...
Bad things are to happening to him because he becomes more and more paranoid about the curse and he begins to truly believe in it.
Point being, when Mike first finds out about the curse, the thought of it will be implanted in his mind. He will start to notice bad things that he usually wouldn't notice before he knew he was cursed. As time goes by he notices more and more bad things. As he notices more and more bad things he gets more and more paranoid about the curse. Thus, making the curse seem more and more real to Mike. Until the curse has actually manifested into Mike's reality.
It's all in the mind. The mind is more powerful than you think. So, if someone says that they have cursed you, just shrug it off because if you let it get to you, the curse may just become real. I truly believe in the power of the mind. The MIND is a VERY powerful thing.