Imam Khomeini explains the matter as following:
Absolute purification from the impurity of polytheism and obtaining perfect authenticity (ikhlas) of devotion is essentially a primary, condition for attaining the stage reserved for awliya’ Allah, and there are even higher stages which they can attain, but, here, it would be out of place to go into these details.
Our Imams (A) have declared that their worship was the worship of emancipated souls (ahrar), which was performed for the sake of love of God alone, neither due to the fear of hell nor in the hope of heaven; and they considered this stage to be the first step of their wilayah.
To them worship is a state of ecstasy and rapture which is beyond the reach of our imagination and understanding. Apart from the above-mentioned ahadith narrated from the Prophet (S) and Amir al-mu’minin (‘A).