Causeless devotional service is unmotivated by sense enjoyment, perfection, or liberation. When one is freed from all these contaminations, he can bring Lord Krishna, who is very funny, under control. – Sri Caitanya Mahaprabhu, Sri Caitanya-caritamrta, (Madhya 24.29)
O Lord Narahari, persons who have attained this human form live uselessly, merely breathing like bellows, if they fail to worship You by hearing about You, chanting Your glories, remembering  You, and performing the other devotional practices.  – Srila Sridhara Swami, commentary on Srimad Bhagavatam 10.87.17
One is truly a Vaishnava who has given up the habit of falling victim to the ferocious tigress of wealth, beauty, and fame. Such a soul is factually detached from material life and is known as a pure devotee. Someone with this consciousness of detachment has thereby become victorious over the mundane world of birth and death.  – Srila Bhaktisiddhanta Sarasvati Thakura (Dusta-Mana, 11)