Yoga Sutra 1.4
vṛtti-sārūpyam-itaratra
Otherwise the seer identifies with the fluctuations
In the last yoga sutra (check out sutra 1.3 here if you missed it), we learned what happens when we practice, what happens when we learn to still the mind. Here, in yoga sutra 1.4, Patanjali continues this thought saying that otherwise, we will continue to identify with the fluctuations of the mind.
As humans we experience an ignorance of our true self, it is normal to identify with our natural world and the thoughts that fill our minds. We confuse our true identity with the things that make up the world around us. Through practice we learn to still the mind, and realize that our true nature is separate from this three dimensional world (sutra 1.3). However, if we do not practice, we will continue to identify with the vrttis (fluctuations of the mind).
Although the goal is stillness of the mind and identification with our true nature, we should not get frustrated when we find that we are identifying with vrttis. We must remember that we are still living in a human form, and identifying with our natural world is essential for survival. We must identify with our work and our loved ones in order to live a productive and fulfilling life. Identification with hobbies and pleasure gives us joy and relaxation. We must work to find a harmonic balance between non attachment and identification with our true nature, and identification with our mind-chatter to exist in the modern world.