Choosing an Ishtadevata is similar to choosing a husband/wife/companion/friend. You ask a person why he or she chose someone as a husband/wife/companion; he or she might give you some answers. But basically you fall in love with someone and then try to justify your selection. We can try to rationalize our decision but cannot do it completely. It is a decision of the heart. It is something called Love. You choose your Ishta Devata out of love. It is a decision of the heart. The intellect does not play a major role in the selection. That is the reason we end up by saying MAA chose me. Don't we end all discussions about the choice of our lover by saying I fell in Love. Period.
But there is a basic difference. Here the love could be Mathru bhava, Pithru bhava, Vathsalya bhava etc, depending upon how you view your Ishta Devata as Mother, father, child, friend, lover etc.
However we can analyze the reasons for choosing a particular deity as Ishta Devata.
1. In most of the cases the Deity whom you have known from childhood is the Ishta Devata. If you have played with Krishna idols in childhood most likely Krishna will be your Ishta Devata. Krishna is I think the most popular God as Ishta Devata as we believe that Krishna is our friend. We look at his pictures as Bala Krishna and feel comfortable with him rather than with Siva who is always a father figure.
2. The family deity is another choice. Again a question of familiarity.
3. It is said that "For a child Mother is GOD." So many people are drawn towards Goddesses.
4. Many people who are devotees of Bagavan Sri Ramakrishna Paramahamsa also become devotees of MAA BHAVATHARINI. Since you read so much about her that over a period of time she becomes your Ishtadevata.
5. Many people are attracted to MAA KAALI because there is no caste distinction in most of her temples. MAA KAALI/ MAA Durga is the favourite Goddess of all revolutionaries because of their association with overcoming the forces of evil.
Recently I was talking to a converted Hindu from Europe. She said that she saw a photograph of a Hindu goddess in a shop and immediately felt that this is her deity.
You always hear of the classical theory of the three gunas. Satwik, Rajasaik, Tamasik. People and gods are again classified according to the gunas. The theory says you choose the gods according to the gunas to which you belong. This theory looks good, but in practice it becomes irrelevant. MAA KAALI is the popular Goddess in Bengal. She is very popular among the so-called lower castes in South India. Do we conclude that all Bengalis and South Indians who worship MAA KAALI are Tamasik or Rajasaik in nature? Was Bagavan Ramakrishna Paramahamsa Tamasik or Rajasaik in nature?
We worship our Ishtadevata as the Ultimate power. This has no gunas. Nirguna. That is why MAA KAALI is called trigunathmikai. So we can safely discard this theory of Gunas ascribed to Gods/Goddesses.
So you choose your Ishtadevata as one whom you fall in love with. When it happens it will happen. Then you can rationalize your decision. As I said earlier the Ishta Devata could mean mother, father, child, friend etc. depending upon your feeling. Psychologists have written so many books trying to explain why we fall in love with someone or why we choose our friends. But I do not think there is a perfect answer. Again the concept of Ishta Devata is basically Tantrik. The Smarthas (I was born one) do not believe in this. But most people have a favourite god. Only that most of the Hindus do not use the term Ishta Devata. Your favourite God becomes your Ishta Devata.
I remember one of friends in a Yahoo group saying that even in spirituality, everything has to be logical. I do not think we can rationalize decisions of the heart. The decisions are not always logical.
It is not necessary to have an Ishtadevata for spiritual or religious Sadhana. But if you look into your heart you will find your Ishta devata. Other than Ishta Devata, you have Kula Devata, Grama Devata etc.