Let us start with an example: You have lot of money and banks will be happy to give you loans because they are sure about your payment ability. On the other hand, if you go to a bank and tell them you have no money but you want a loan, you are more likely to be rejected.
Nature seems to work in the same way. When you say 'I'm happy, you are more likely to get more happiness. If you feel sad, you will also attract more of that. So the key is in maintaining I'm happy attitude all the time.
