Firstly, it underscores the importance of this person in the speaker's life by using powerful terms like "best friend," "soul mate," and "everything." These words convey that the person is not just a romantic partner but also a cherished and integral part of the speaker's existence.
Secondly, it conveys the idea that losing this person would mean losing not only a romantic partner but also the source of joy, happiness, and completeness in the speaker's life. Their presence is associated with laughter, smiles, and a sense of wholeness.
In essence, this statement is a testament to the depth of love and emotional connection between the speaker and the person they love. It signifies that this individual is not just a romantic interest but the cornerstone of their happiness and fulfillment, making the thought of losing them unbearable.