In this world everyone is born into the darkness of ignorance (om ajnana timirandasya) which relates to the Christian concept of "Original Sin."  This state of being is predominately marked by our forgetfulness of our true spiritual identity, to which we must rely upon God to awaken us either directly Himself or indirectly through Guru.  As Krishna says regarding his own appearance:

Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all living entities, I still appear in every millennium in My original transcendental form.  Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion-at that time I descend Myself.  To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.

Bhagavad-gita 4:6-8 (Prabh)

When God is not directly available as Guru, Krishna recommends:

Just try to learn the truth by approaching a spiritual master.  Inquire from him submissively and render service unto him.  The self-realized souls can impart knowledge unto you because they have seen the truth.  Having obtained real knowledge from a self-realized soul, you will never fall again into such illusion, for by this knowledge you will see that all living beings are but part of the Supreme, or, in other words, that they are Mine.

Bhagavad-gita 4:34-35 (Prabh)

Worshipping our guru means revering anyone who gives spiritual instructions.  It does not mean worshipping only that person who gave you initiation; this is the difference between Siksa and Diksa gurus.  The Diksa Guru is the teacher that initiates one, gives one the vows of sannyas (ordination into full monkhood).  The Siksa Gurus are all the people, self-realized or not, that instruct or teach us lessons.

Why worship, respect or revere the guru?  Just as there is considered to be no difference between the guru and the instructions of the guru, we revere the guru as being the representative used by God to awaken us from our slumber of ignorance.  Since these lessons can come from anyone, we accept even a non-liberated person as being able to be used by God as guru, hence the title of Siksa Guru.  Also, the vanu (words) of the guru are considered to be more important than vapu (his personal presence) since they continue to exist, even when the guru is gone.