Humans have evolved to realize that performing certain actions which are tough in the present moment, but lead to great rewards in the future. Also, it has been observed that hankering after results also leads to several problems.
Hence performing actions without the anticipation of results appears to be the best approach.
As Krishna delineates the three ways to perform actions, penances etc, he categorizes those actions performed for their own sake as belonging to the the Sattvika category.
aphalākāṅkṣhibhir yajño vidhi-driṣhṭo ya ijyate
yaṣhṭavyam eveti manaḥ samādhāya sa sāttvikaḥ
This can be roughly translated to:
That sacrifice which is offered by men without desire for reward as enjoined by the ordinance (scripture), with a firm faith that to do so is a duty, is Sattvika or pure.
– Bhagavad Gita 17.11