comment: >
  
  Did Kant ever expand on your question?
  It seems rather obvious to me that doing good things feel good. The question you float can only work once. After you feel good for doing good the first time, have you spoiled your good will? You will never be able to perform truly selfless acts because you have a memory of feeling good that first time?
  That seems rather silly and not well reasoned to me
context: This kinda begs the question...   
  What does it mean for an act to be selfish? >
  
  Oh dang I'm not sure... maybe done without morality? Or done for the benefit of oneself? Both occurring simultaneously? >
  
  Most people would say that selfishness/selflessness is defined by the intended beneficiary. That means that the same action can be either selfish or selfless depending on the intent.  
  For example, giving to charity can either be selfless (because you aren't considering your self in the decision - you're primarily thinking about the wellbeing of others) or selfish (because you're doing it so you can boast to your friend about how much of a good person you are).  
  This is a non-consequentialist (not based on outcomes) form of morality.  
  The way that Immanuel Kant explains this problem is by defining what he calls "good will", which is the test for whether an action was moral. He says that an action taken with good will means that your only consideration when taking the action was the intent to abide by moral law. What that means in simple terms is that whether or not it's selfish is defined by what you were thinking about at the time you took the action. If you were thinking exclusively about your moral obligation to help others, then it's a moral action, but if you paid any attention at all to your own happiness then it was not purely "good will" motivating you. In this way, Kant would agree with you that just because an action made you happy, it's not necessarily selfish.  
  Non-consequentialism isn't the only type of ethical evaluation though.  
  For instance, a naturalist is someone who believes that our moral sense was invented by evolution to help us co-operate with others. From a naturalist point of view, all actions made with the intent to abide by moral law are actually selfish actions, because they invoke the biological desire to take actions which increase your likelihood to reproduce. A utilitarian (someone who thinks we should maximize the total amount of happiness in the world) would reject the entire classification of selfish and selfless because all that matters is the change in the total amount of happiness.   
  The question I would float is whether you had awareness that taking the action would improve your happiness before you did so. If you were aware it would make you happy, it is therefore impossible to possess purely good will as defined by Kant. If you didn't know, it's not possible for an action to be selfish because you didn't have access to the information required to act selfishly. It's not necessary that an action that makes you happier was selfish because it's possible to be unaware of that occurring, but in the vast majority of cases humans are good at predicting their future emotions and reactions to things in simple ways.