Nature has given the human beings the power to create and destroy Chittavritti-s. Chittavritti-s are the actions or movements in the mind. Most of the human beings are not able to control the Chittavritti-s. Some people can control some Chittavritti-s. Through actions and thoughts, or in other words, actions of the body and mind, Chittavritti-s are created and subdued. This increase and decrease of Chittavritti-s in the human mind is the reason for the discovery of the system of Yoga.
The instinctual beings like animals and birds need not do any Yoga for their happiness because they never create unwanted Chittavritti-s in their mind. They live according to the laws of Nature. It is when you break the law of Nature that you need to come back to it. Human beings break the law of Nature by creating unwanted Chittavritti-s in mind. The desires for illogical pleasures are the reason for the unwanted Chittavritti-s. For satisfying the lust, human beings go beyond the Nature, which is the reason for all unhealthy tendencies in human mind. The unhealthy Chittavritti-s for each one varies according to his/er situation. For subduing the unwanted Chittavritti-s and to develop positive Chittavritti-s, human beings are needed to practice Yoga.