This duty thing is very vexing. I mean, if there is any saying that is much over-used (AND most popular) from the Bhagavad Gita it is...YES! THAT is it! 'Karmanye vadhikaraste ma phaleshu kadachana.' OR, in English, 'Do your duty, look not to the fruits thereof' or some such. See, it is not enough that this duty thing involves DOING, instead of lolling around, but you are also supposed not to expect anything out of it.
About the only thing that keeps your nose to the grindstone is the fact that, at the end of it, you'll benefit from it. AND here these guys come, telling you that you are not to expect anything out of it at all. And Tiru is certainly no ray of sunshine when he talks of the same subject.
Dhavam seyvaar than karuman seyvaar matrallaar avan seyvaar aasai utpattu - Tirukkural
Only he whose mind is free of desire truly does his duty; the rest waste their efforts being distracted by desires - Loose Translation
You know, these words like 'Dhavam' are a tough nut comes to translation. People sort of call it 'meditation' and dust their hands off thinking that they have done a good job. Not really. Dhavam is a mix of worship, the mindset of meditation, the renunciation of a sage, yada yada. Which is one of the reasons why the West finds it impossible to GET Indian philosophy - the fallacies of complacent translators. In the instant Kural, the mind free of desires IS the most apt attribute to use for Dhavam.
Where the Gita tells you to avoid concentrating on the fruits of your actions, it is seen more as a philosophical construct. That if you do anything as a duty, without being driven by the desire for the fruits of the action, you are free of the sins of that action. (AND, yes, that 'duty' IS an imperfect translation of 'Dharma'. Every human being is supposed to adhere to multiple dharmas - as a human, son, a husband, a neighbor, a citizen, an employee etc. etc. SO, 'duty' as in Indian philosophy is not the simple thing of saying that "I was obeying orders therefore I am free of sin"! There is a hierarchy of dharmas to live by, not just one. Especially when you are supposed to acted while being free of desire for the fruits - promotions, desired transfers OR even escaping being shot at dawn.)
Tiru is not merely being philosophic here. What he, in effect, is saying is that you WILL be doing whatever you set out to do only if you free your mind from desire. If your mind is always set on your desires, then your effort will go waste because you are not taking interest in the job itself. Sweeping assertion, yes, but is true to a greater or lesser extent in all jobs. How good a job you do of ANYTHING is a function of how much interest you take in doing it. And if your desires rob your concentration on the job, the results are likely to be less than good.
What, then, if you conceive of YOUR job AS satisfying your desires and nothing else? As most of us seem to do. Point to ponder? Not really IF your desires are to be satisfied by others and not exclusively by yourself. THEN your primary objective IS to satisfy those others, even if the final goal is to satisfy yourself. Therefore, you still need to do a good job which means you need to focus ON the job, and not on your desires, at least WHILE you are doing it.
Sad! No philosopher ever seems to project a decent shortcut to success. What's the use of philosophy if I still have to work, I say!