As per my understanding, Guru is a special agent of God.
In response to our incessant petitions, God has appointed or commissioned Guru, his closest friend, to help his children. He may appear to be an ordinary individual, but he is not. He is so beloved by God that he is the embodiment of God himself. God works his magic through his special agent, the guru, and therefore God's love flows effortlessly through Guru to disciples.
It is said, "The blind cannot lead the blind". Only he who knows god can lead us to him. Guru is the most devout follower of God. So, even though he is god realized, he took it upon himself to rescue us, the god's children.
Guru came to earth in human form to show us how to travel on the path that leads to God. In human form, he faces the same struggles that we all do. He endures it all with a smile on his face, keeping God's presence in his heart, for our sake. Even if he is going through human life's ups and downs, he takes great care of his disciples' day-to-day issues.
Guru had promised God that he will bring his children to him. He will never abandon his children or break his promise to God. He will take numerous births if necessary, but he will never abandon a single child. His only purpose is to free us from the bounds of life and death and guide us to salvation.
Being a Guru isn't easy. It comes at the cost of great self-sacrifice. It entails giving up his own aspirations to help realize the dreams and betterment of others. He sacrifices his personal preferences, and his own personality as a human to help his disciple.
Guru reaches out his hands to each of us. We may be hesitant, but he remains patient with everyone. We may not remember him, But he remembers us from this life and many others.
His love for us is unconditional, unchanging, and eternal. He cares deeply even for the disciples who are no longer with him.
He does not care for personal glory or popularity. His sole motivation is to help his disciple progress spiritually. He is close to each of his disciples. He has individual connections with each of them. His presence goes much beyond the physical body he lives in. He is ever present with each of his disciples.
When we are attuned to Guru, we are attuned to God.
Whatever little I know about God, I know it through Gurudev. Gurudev is the fountain through which God's love has reached me. He is the window through which I have caught glimpse of divine.