Without Guru, there is no way of knowing God. This is the reason even when Sri Krishna comes down to this world, He takes a guru to show us as the example.
However, unfortunately in this age of Kali, this world is flooded with false gurus who have their own agenda based on self interest. But if you have a honest desire to surrender to a Guru and receive instructions, Sri Krishna says that He will send an appropriate guru. The guru we get depends upon our genuine interest. If we have sincere devotion to Krishna, then Krishna sends a real guru. We don't have to look for a guru, but guru appears when a student becomes ready!.
One may ask that why many people do not want to surrender to a guru? The reason is their EGO. So, ego plays an important role in accepting a spiritual master. They may ask, 'Why should I need a Guru?" Can't I go myself? The answer is that because we have fallen down to this material world, we need help from some one who is positioned in the spiritual world. For learning music, we look for a good teacher. To learn tennis, we look for a teacher. For medical problems, we go for a doctor, sometime even for second opinion. But when it comes down to spirituality, they say that they can do by themselves. Unfortunately, self service does not work in this case. How many of us learned our ABCD by ourselves. How can a frog fallen deep into a well jump out by itself?
We may wonder as why there is a middle man necessary to worship the Lord. We may even wonder if this system had been invented for the sake of some sect. But the fact is that every parampara, every acharya, and every sastra proclaims that one should need a guru. Lord Krishna explicitely says that if any worship is done to Him without first worshipping one's guru is not accepted by the Lord.
Prathaman tu gurum pujyam Tatas caiva mamarcanam Kurvan siddhim avapnoti hy Anyatha nisphalam bhavet (Hari Bhakti Vilasa 4.344) "One must always worship one's guru first and only then worship Me (Lord Krishna). When one follows this gets perfection, but if one neglects this process, everything will be fruitless."
Also accepting a guru is not just a formal thing. In many cases in the history, a formal initiation was not even performed. When one genuinly surrenders and accepts a guru with full heart, that is the real initiation.
When one accepts a guru, he is called a dvija - second born. Our first birth is from our parents, and the second birth is by initiation from a guru. Dvija is second born. A bird is second born, as its first birth is an egg, and the second birth is its hatching as a bird. According to the sastras, by birth every one is a sudra. 'Janma Jayate Sudrah'. So, brahmanic qualification is never by birth.
Even of one was born in a family of dog eater, if he becomes qualified he is a brahmana and eligible to read the Vedas. he is thus called a Vipra. Then when he accepts a guru, he becomes a brahmana. In this kali yuga, we see many people who are born in brahminical families eat even meat!
Guru and Krishna work together to help a Jivatma to attain its highest destination. They work so together that the sastras say that if one worships Sri Krishna he gets Sri Guru, and if he worships Sri Guru he gets Sri Krishna. "Krishna Krpa Guru Miley, Guru Krpa Krishna Miley" (CC).