If one is not attracted by the transcendental nature of Krishna, one is sure to be attracted to material enjoyment, thus to become implicated in the clinging network of virtuous and sinful activities and to continue material existence by transmigrating from one material body to another. Only in Krishna consciousness can one achieve the highest perfection of life.
His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Sri Chaitanya-charitamrita, Adi-lila 4.35, Purport

What the Upanishads describe as the impersonal Brahman is but the effulgence of His body, and the Lord known as the Supersoul is but His localized plenary portion. Lord Chaitanya is the Supreme Personality of Godhead, Krishna Himself, full with six opulences. He is the Absolute Truth, and no other truth is greater than or equal to Him.
Srila Krishnadasa Kaviraja Goswami
Sri Chaitanya-charitamrita, Adi-lila1.3

Devotional service is the propensity to give Krishna pleasure. The endeavor to make oneself happy is nondevotional service and is the source of misery.
Srila Bhaktisiddhanta Sarasvati Thakura
Amrita Vani: Nectar of Instructions for Immortality

Whatever a self-realized soul in India will speak, a self-realized soul in another part of the world will speak. A devotee in Vaikuntha will also speak the same thing, because there are no material qualities in the conclusions of pure, liberated souls. Therefore the conclusions cannot be different.
Srila Bhaktivinoda Thakura
Tattva-viveka, Chapter 1

Persons who seriously follow these methods of achieving Me, which I have personally taught, attain auspiciousness in the form of My personal abode, which authorities understand as the Supreme Absolute Truth.
Lord Sri Krishna
Srimad-Bhagavatam 11.20.37

Of all the eternal living entities, one [the Supreme Personality of Godhead] is the supreme eternal. Of all conscious entities one [the Supreme Personality of Godhead] is the supreme conscious entity who supplies the needs of everyone else. The wise souls who worship Him in His abode attain everlasting peace. Others cannot.
Shvetashvatara Upanishad 6.13 and Katha Upanishad 2.2.13

The Supreme Personality of Godhead is the source of genuine happiness. Nothing else can bring one actual happiness. Only the Supreme Personality of Godhead can bring one happiness. For this reason one should inquire about the Supreme Personality of Godhead.
Chandogya Upanishad 7.25.1

Distress is caused by five things: nescience, false ego, material attachments, envy, and mundane engrossment, all of which essentially represent different aspects of the mode of ignorance.
Srila Vishvanatha Chakravarti Thakura
Madhurya-kadambini, Second Shower