Why Bhakti?
The innate nature of Human or any animal is to establish authority over the other entity within its per-view. If you put two people in the room, they try to establish the superiority over the other. Humans are constantly working on having that sense of control to establish their existential identity.
When the idea of Supreme being, all knower, all powerful, the concept of GOD established in Human mind, now how do you get on to its side and how do you overpower the GOD itself. This is the ego function of the beings. The idea of Devotion emerged. Where we think we can bind God this almighty with our devotion. He is at the beck and call at the devotee. Protecting, providing, full-filling the wishes of devotee.
You will see umpteen examples of this, where in the past mythologies especially from India, people did TAPAS (penance) to please GODS, to acquire certain boons, capacities and powers. Some of the Rakshasas(demons) once they acquire the Siddhis(powers), even tried to go against the same Gods who bestowed them those powers.
This is existential struggle of the Human being at mental, physical, emotional, intellectual pushes humans into a compulsive state. Today most of the so-called religious sentiments fall in this category.
On the other hand, our seers of the past, the Rishis who contemplated over this said Bhakti cannot be an Ego function, it has to be somewhat more profound and deeper than this. It cannot be bounded or expressed within the context of human emotion and sentiments.
Then why bhakti or what is bhakti? As human consciousness expanded, a realization dawned upon the being that if this whole thing created from some state of nothingness, a true divine essence, then it must be all pervading. It must be within me and outside me. Now to get to that essence, which is not tangible by any means, only solution is to become as subtle as the essence to have a grasp of it. As mind is constantly engaged even if one gets the glimpse of this essence through meditation can be lost in no-time. Hence, they said by constantly remembering that state of essence will slowly become every expanding, constantly changing state, mind cannot be devoid of it. This constant remembrance is nothing but Bhakti.
As one remembers shall become that. Hence its a constant expansion of this consciousness to reach this infinite state. There is no ego function here, its just being and becoming.
All asking, desires, approach to life, ego, emotions, all lower faculties of the mind attain certain balanced state. Its like when we are thirsty, take a cup of water there is a sense of balance. Whenever any need is fulfilled creates certain balance in the system.
When mind attains this Bhakti, state of constant remembrance, all faculties come to balanced state. All the compulsive nature is gone, the need for existential struggle disappears.
There are so many ways to see the ideologies for example, Advaita is defined as NOT TWO. Commentators have concluded NOT TWO means ONE. But at the culmination of Bhakti neither the devotee nor the object of devotion exists, both disappear. NOT TWO truly means neither that nor this both don't exist. You see it’s a complete annihilation of the Ego, existential identity.
Swami Vivekananda said Bhakti at the lowest leads to fanaticism. What is this lowest state is he referring to, is the state of consciousness which tends to associate with the forms without trying to grasp the subtleness of the essence, this approach cannot lead to the expansion of consciousness. How can we grasp the infinite state from a contracted consciousness. It limits our approach. Our protective sense of existential identity will not allow so easily overcome this barrier, as this feels like total annihilation.
For the same reason one must subject to the practices which remove this barrier and allow it to expand.