The whole idea is everybody wants to be happy (including yours truly) and thus with that in mind, we do what makes others happy first and in return, we become happy. If our first concern is self, this may lead to grasping which will then lead to unhappiness. So best to give first then take later.
Giving first , I dispute that somewhat , perhaps some can give first and some can give later. People who are more happier will be more likely to give first IMHO.
To push the idea a bit more, if all of us are interdependent there is no difference between yours and my happiness : ) . There is no separation between my happiness and yours .
Personally I feel that giving back or practicing only when you feel happy is quite shallow. If we are capable we should always try to use the POV that Wai Meng has pointed out. On a deeper level, happiness and pain are just both ends of the spectrum so in reality there's not much of a difference. When you realize that your cravings will lessen a bit..