What is the cause of misery in animates and especially in a human being?
Before heading to understand the concept of misery in the beings, one has to know primarily, the cause of evolution and its merits. The universe is manifested out of the thought of Non-Existent Energy (satwa), called Supreme. The thought (which is the identity of that Supreme as a manifested Universe consisting of animate and inanimate) is the output of His inherent potential energy, that triggered in Him. As a result of such trigger the Non-Existent developed as passion (rajas) and ignorance (tamas), which are the attributes or eminence (Mahat) of His latent energy.
Passion is one of the two attributes of the latent energy that has developed further its attribute into six wise qualities viz. Aspiration, Divergence, Novelty, Awareness, Refinement and Personality, for the creation of rest of the worlds. These six wise qualities (Mahat) are later found to be the vicious qualities viz., Desire, Anger, Greed, Attachment, Pride and Jealousy (arishadvarga) in Jivas due to the power of influence of ignorance on the Jiva (tamas), which is the second attribute of the latent energy of that Non-Existent.
Each one of the wise qualities developed into further categories. For instance, ‘aspiration’ of the latent energy formed the class like ambition, hope, want, etc. Similarly, the rest of the qualities also gave rise to different sub-classes. The effects of these resulted in differences which deceptively appeared as high and low, good and bad etc., in the nature of the creation. Although the differences are ostensibly perceived by a Jiva and not by the animals and inanimate, when the trigger terminates in the Supreme, all the differences of perception vanish along with the manifested universe. And ceasing thus His identity, He goes back to the state of Non-Existent Energy. This is the secrecy behind the curtain of this manifested Universe.
The apparent differences of perception in the nature, influences the life of a Jiva and prevents the Jiva from self realization. A Jiva, who is influenced by the power of ignorance, fails to perceive the things with a right discrimination and becomes subject to misery, whereas, the discriminatory faculty being absent in other animates viz., insects, birds and other animals, they are enhanced naturally [to the higher state of Jiva] in their next birth. The inanimate of course, is manifested out of the Supreme thought, since it is inert in its nature like the Non-Existent Energy, hence, it needs no enlightenment or enhancement and will also cease along with the cessation of thought of the Supreme.
It is the Jiva in His entire creation, who has to undergo the process of refinement in the life cycle. Hinduism, believes in rebirth and in order to egress oneself from the vicious life cycle, a Jiva approaches an ideal Guru/preceptor, who takes charge to direct and uphold his pupil in the right path and ultimately imparting to him the knowledge of realization (merging with God).
From the day the Jivas descended on this universe they have been making their best efforts to understand the cause behind the appearance of this cosmos and their existence in it, but, till this day their attempt to discover the mystery has proved futile. Science undoubtedly brought advancement in the living standards of the beings, yet it failed to discover completely the Author on whose direction this universe appears and disappears. When Jivas understand His cosmic drama and their role in the play, they will completely overcome the negative qualities of the attribute called passion (which is the cause of those vicious qualities) and live in a true state of bliss as non-existent energy (even while being in the body), discharging the deeds in dispassion.