Some time ago a friend of mine (I will call him Phil) did a good deed for someone else. Initially this made him feel very happy himself. But then he started to worry :

"Did I do this for the recipient, or was it largely a selfish act for the pursuit of my own happiness ?"

So he asked a friend (I will call him Dave) for advise, what did he think about it. Dave answered that indeed it was largely an act of selfishness because he had done it to improve his own happiness.
Phil then went to see John and asked him the same question. John immediately replied that Phil had been entirely unselfish with his good deed. Of course he would feel happy afterwards for having given someone else's life a positive or happy lift.

Well, who supplied the correct answer to Phil, was it Dave or was it John ?   The replies do not really provide a reliable answer to Phil's dilemma. They are instead a litmus test revealing the nature of the repliers themselves.

• Dave, who considered Phil selfish is by nature selfish himself. His answer is derived from his own perspective. (Or possibly he never has thought the proposition through properly.)
  
  
• Likewise John's answer, who considered Phil's act as being unselfish, is also a reflection of John's own nature, not necessarily of that of Phil.

In other words, Phil can only answer his question himself.   But is there any tell tale sign he can rely on ?
I believe so. The very fact that he started to worry about his motive is (I believe) a clear indication that his good deed was indeed an act of unselfishness. If he had done it out of selfishness he would never have worried about his motive at all.

Although I only know about this theory from what I have read in the Wikipedia (so please keep this in mind), it is tempting to view the above from the perspective of Bjorn Grinde's theory of Darwinian Happiness.

As I understand it many (if not all) of our feelings are in fact (biological) emotional levers nudging us into directions which benefit ourselves or the propagation of our species (or away from conditions which may harm us or adversely affect our species).
If, regardless of our personal nature (being either selfish or unselfish), we become happy when doing a good deed towards someone else, this emotion (within the context of Darwinian Happiness) is therefore acting as a double edged sword :

• For on the one hand it is the reward for an unselfish person's good deed,
  
  
• while on the other hand it provides the motivation for the selfish person's act of the same.

Conclusion ?
No matter what motivates you to do a good deed towards someone else, it is a positive act which contributes (in whatever small way) to the perseverance and positive development of our species as a whole.