SB 7.7.42

sukhāya duḥkha-mokṣāya
 saṅkalpa iha karmiṇaḥ
sadāpnotīhayā duḥkham
 anīhāyāḥ sukhāvṛtaḥ
Word for word: 
sukhāya — for achieving happiness by a so-called higher standard of life; duḥkha-mokṣāya — for becoming free from misery; saṅkalpaḥ — the determination; iha — in this world; karmiṇaḥ — of the living entity trying for economic development; sadā — always; āpnoti — achieves; īhayā — by activity or ambition; duḥkham — only unhappiness; anīhāyāḥ — and from not desiring economic development; sukha — by happiness; āvṛtaḥ — covered.
Translation: 
In this material world, every materialist desires to achieve happiness and diminish his distress, and therefore he acts accordingly. Actually, however, one is happy as long as one does not endeavor for happiness; as soon as one begins his activities for happiness, his conditions of distress begin.
Purport: 

Every conditioned soul is bound by the laws of material nature, as described in Bhagavad-gītā (prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ). Everyone has achieved a certain type of body given by material nature according to the instructions of the Supreme Personality of Godhead.

īśvaraḥ sarva-bhūtānāṁ
 hṛd-deśe ’rjuna tiṣṭhati
bhrāmayan sarva-bhūtāni
 yantrārūḍhāni māyayā

“The Supreme Lord is situated in everyone’s heart, O Arjuna, and is directing the wanderings of all living entities, who are seated as on a machine, made of the material energy.” (Bg. 18.61) The Supreme Personality of Godhead, the Supersoul, is present in everyone’s heart, and as the living entity desires, the Lord gives him facilities with which to work according to his ambitions in different grades of bodies. The body is just like an instrument by which the living entity moves according to false desires for happiness and thus suffers the pangs of birth, death, old age and disease in different standards of life. Everyone begins his activities with some plan and ambition, but actually, from the beginning of one’s plan to the end, one does not derive any happiness. On the contrary, as soon as one begins acting according to his plan, his life of distress immediately begins. Therefore, one should not be ambitious to dissipate the unhappy conditions of life, for one cannot do anything about them. Ahaṅkāra-vimūḍhātmā kartāham iti manyate. Although one is acting according to false ambitions, he thinks he can improve his material conditions by his activities. The Vedas enjoin that one should not try to increase happiness or decrease distress, for this is futile. Tasyaiva hetoḥ prayateta kovidaḥ. One should work for self-realization, not for economic development, which is impossible to improve. Without endeavor, one can get the amount of happiness and distress for which he is destined, and one cannot change this. Therefore, it is better to use one’s time for advancement in the spiritual life of Kṛṣṇa consciousness. One should not waste his valuable life as a human being. It is better to utilize this life for developing Kṛṣṇa consciousness, without ambitions for so-called happiness.