Why do we do any work, either for self or for others, but sometime some work happen because it has to happen, when the result of the work is not as per our desires we become unhappy and sad while if it is as per our wish we are happy, but this is temporary, if we wish to achieve permanent happiness then we have to understand the series of karma to be implemented for that particular work,
When Karma is with responsibility, it becomes our duty, every karma is not duty, only those karma becomes your obligation or duty with whom you attach your character, life policies, values and art of living,
Keep your duty connected with love and devotion, because when duty is connected with love, there is a feeling of belonging, for whoever you are doing you duty, the love makes you feel its your own karma for self, and we say within, "its ok if I did something for them, after all they are my loved ones"
On the other hand, if your duty is connected with devotion and devoted to God, then you are connected to God, and we say, "i have to do this no matter what, I am not worried about results because God knows what’s best for me"
From above two points, when we practice, we unknowingly remove "I" or "I did" and from here starts the permanent happiness, to be happy always, connect your work with love and devotion and you need not to worry ever,