One who is not envious but is a kind friend to all living entities, who does not think himself a proprietor and is free from false ego, who is equal in both happiness and distress, who is tolerant, always satisfied, self-controlled, and engaged in devotional service with determination, his mind and intelligence fixed on Me—such a devotee of Mine is very dear to Me.
(Bhagavad-Gita--12:13-14--translation).

Only the self-controlled can gradually be attached to the Supreme Personality of Godhead. Self-controlled means not indulging in sense enjoyment more than is necessary. And those who are not self-controlled are given over to sense enjoyment. Dry philosophical speculation is a subtle sense enjoyment of the mind. Sense enjoyment leads one to the path of darkness.
(Srimad Bhagavatam--1:18:22--purport).

Self-control is actually achieved not by artificially stopping the senses from material enjoyment, but by becoming factually attached to the Supreme Lord by engaging one's unalloyed senses in the transcendental service of the Lord. The senses cannot be forcibly curbed, but they can be given proper engagement. Purified senses, therefore, are always engaged in the transcendental service of the Lord. This perfectional stage of sense engagement is called bhakti-yoga.
(1:18:22--purport).

By self-control one can be free from false hopes, and money can be saved by avoiding undesirable association.
(Srimad Bhagavatam--1:9:27).

Steadiness means that one should be very determined to make progress in spiritual life. Without such determination, one cannot make tangible progress. And self-control means that one should not accept anything which is detrimental to the path of spiritual progress. One should become accustomed to this and reject anything which is against the path of spiritual progress. This is real renunciation.
(Bhagavad-Gita--13:8-12--purport).

One who is not self-controlled and whose mind is not undisturbed cannot practice meditation. Therefore, in the Bṛhan-nāradīya Purāṇa it is said that in Kali-yuga (the present yuga, or age), when people in general are short-lived, slow in spiritual realization and always disturbed by various anxieties, the best means of spiritual realization is chanting the holy name of the Lord.
(Bhagavad-Gita--6:11-12--purport).

By transcendental knowledge one can remain steady in his convictions, but by mere academic knowledge one can be easily deluded and confused by apparent contradictions. It is the realized soul who is actually self-controlled, because he is surrendered to Kṛṣṇa. He is transcendental because he has nothing to do with mundane scholarship.
(Bhagavad-Gita--6:8--purport).