Attentive chanting leads to meditation on the Lord’s qualities, form, and pastimes. If we chant with a deep intention and eagerness to listen attentively, gradually we will appreciate the Lord’s qualities, then His form, and eventually, His pastimes. When each of these three aspects of the Lord’s form manifests, they will become part of our meditation and we will be able to remember them while we chant. But we cannot force these aspects to appear. That is why Srila Prabhupäda emphasizes the importance of attentive hearing while chanting. If we can simply learn to hear the holy name, the mind will be clear and stable.
(LFD, Vol2, Chanting Purely, Pg 106, 2nd paragraph)