In the sun there is great heat and light, and by reflecting the greatness of the sun’s light, the moon in night also appears great. Actually the moon is by nature dark ans cold, but in association with sun it has become accepted as great. Hence by association of the Great, one appears great.
Good association means association with one who simply accepts the Krishna as his worshipable deity.
If the devotee feels that, "I can be Krishna Conscious without the association of devotees." it is very dangerous situation. More we separate from devotees, more we bounded by the attacks of Maya. The material world is designed to struggle to exist. So, Srila Prabhupada made the situation minimum by having Krishna conscious temples, communities and farm projects, so devotees can live in association with devotees.
When we associate with devotees we slowly-slowly get to know about their great qualities and thus we give up our envious nature.
In Chaitanya Caritamrta Lord Caitanya says that sadhu-sanga, the association of a great saintly person, is very important, because even if one is not advanced in knowledge, simply by association with a great saintly person one can immediately make considerable advancement in spiritual life.
A devotee should always aspire to live in the association of advanced devotees and engage in the service of the Lord through the parampara system.