Perhaps one impediment to accept a guru is one's pride, and inability to realize the need for help.
Second is when one develops a relationship with one's guardian angel, and takes him/her (angels are genderless, but we just can't call an angel "it".) as God. Christianity especially suffers from this, based on the judean tradition about one God. So when one expereinces something spiritual, one takes that as an expereince of God, while it might be an angel, a devil, another human beings astrality and so on. So many experience bliss and peace and they think they achieved the ultimate, while they are just slaves of a higher form of illusion. The possibility for such errors is huge. The avearge person is protected from these, but from one who follows a spiritual discipline this protection is withdrawn ... because that's what he/she is asking for.
A guru is a must. There is the saying: repetition is the father of knowledge. The guru is there to provide sure ground. He will say the same things for years ... and what is funny, the disciple will comprehend it more and more, and new depths will be revealed in time to these apparently simple teachings.
A guru must not necessarily be a human being, though. Or rather, I'd say, must not be necessarily present on the physical plane. In fact there are many spiritual teachers among the dead, we just need to become able to listen. The spiritual world is saturated with wisdom, we just need to receive it. But because of this abundance, a guide is necessary. Otherwise, what happens to the disciple is similar to what happens when we die. When a person dies, who did not cultivate any interest towards spirituality, the experience is similar to drowning. After death, we can only relate and benefit to those spiritual powers whom we met, get used to, learned about during the physical life. This is not necessarily the information in our heads based on some theology, although that too helps a little. It is rather like those moments of awe of our lives, when some experience touches us deeply, when we get a glimpse into the eternity of the spirit.
To accept the need for help, to become humble, is not in contradiction with human dignity, or freedom. Real dignity cannot be threatened, and real freedom can't be taken away. Until we fear for our dignity, or freedom, it is clear we do not posess them yet !
Humility is realising the limits and origin of our ego, and accepting them. Than, we realize that the selfishness of the ego, bursts up form the very core of our being, and it directs our actions both conscious and subconscious. And until we are unable to look into this place, this abyss, we will never develop the strenght to turn our eyes into another direction. Because of this, any initiation, enlightment, goes through the death of ego, through death, and it is a trying expereince. Without this death of the old man, the new Man cannot be born, the spirit Self cannot be realized.
What better way to realize this Self, is than to say: Everything I think and know I am, dies, is ended, falls, is terminated ... and through this dispair, sadness, of nothingness, loneliness and hoplessness, you will realize: I still exist ! I AM ! The son of man needs to die, so the son of God can be resurrected in us.
Yoga is not that different ... (pratyahara, dhyana, samadhi ... because what are before these are just preparations) but what once was a secret initiation, through the Deed of Christ, the Cosmic Self, became the Way of many. At least, if correctly comprehended.
It really does not suffice to say, through Jesus I am saved. That is meaningless, and just talking. Rather, a real christian, like the real yogi, dies, so he/she receives eternal life. The yogi performed his death in seclusion, in his innermost being, the christian performed his death on the bloody floor of the roman Colisseum. The yogi saved himself but through his/her example, the christain saved many.
Today, too, those are the real yogis and christians, who can bring the sacrifice of their ego to the altar of Spirit. They know that they are not fools, but they trade the passing, the decaying, for Life. The very nature of Life is that it lives ... and never dies.