1) The soul exists because
a) All aggregates of components are for the sake of another.
For example in a house cot, chair, sofa etc are available. This indicates that there is a user and they are subservient to a user. All composite things are ultimately for a non-composite thing that is unitary-soul. A corollary to above is that soul does not have attributes. Only a compound entity will have Gunas.
KRISHNA enumerates attributes of soul in various verses especially verses 13 to 17.Entire Gita leave alone chapter 2 is based on this.
2) There must be a controlling agency. Mind supplies to intellect (buddi) various impressions. Buddhi identifies pleasure, pain and delusion. This is submitted to the controlling agency which is final. Why should it be final? This is to avoid finding another agency and there will be no end to it. This avoids regressus ad infinitum.
3) Another reason is that things of enjoyment (bhogya) are the visible things like the intellect and the rest. They cannot become visible without an observer. There is a seer beyond visible things and that is the soul. THUS THERE IS AN EXPERIENCER OR OBSERVER.
4) Another reason is the concept of emancipation. Emancipation implies final cessation of pain and suffering resulting in BLISS. This emancipation cannot take place in buddhi or mind or other instruments. Why? This cannot take place in composite things. This will take place only on an entity which is simple and non-agent. This entity got entangled and hence can aspire for emancipation. So we find activities of sages gear towards the emancipation of soul.
Thus existence of the soul is established .But the question remains whether the soul is one or many. Here opinions differ. But that is a complicated subject. It needs special study. Also the relationship between soul and God is open to debate between various schools of thought. The soul has only one attribute-if we may say so- that is it is a pure witness, neutral, and non agent.In fact at higher level it is not even doing witness function becasue that is also an act.Strangely enough the soul or Atman is not working but it’s presence causes work.
Soul is a non-matter and hence does not suffer from modifications which matter or Prakrit is subject to. For example matter is acted upon by physical laws such as laws of motion, thermodynamics etc, BUT SOUL IS SIMPLY BEYOND ALL THIS. KRISHNA enumerates ideas of soul in various verses especially verses 13 to 17, Entire Gita leave alone chapter 2 is based on this.