It seems that, initially, Victor is so caught up in pushing the limits of scientific boundaries that he is quite blind to the reality of the creature he is creating. In order to accomplish this feat, Victor "collected bones from charnelhouses and disturbed, with profane fingers, the tremendous secrets of the human frame." Yet after working tirelessly on the creature for two years, Frankenstein doesn't see the reality in front of him until he actually sees the success of his efforts—it "breathed hard." Suddenly, Victor realizes the "catastrophe" of the situation. This is no beautiful creation. It is a compilation of various body parts which he has robbed from other sources and therefore is a "demoniacal corpse to which [he] had so miserably given life."

There is certainly shame in the decision to remain quiet—a shame rooted in the acquisition of materials he has used. There is also horror in seeing this grotesquely assembled being actually breathe; perhaps even Victor didn't think he could actually pull this off. The combination of horror and shame drives Victor to abandon this creature.

But then another event complicates the situation even further. With no one to guide his moral compass, the monster lashes out at Victor by killing his youngest brother, William. Now Victor is in a true conundrum; if he tells anyone that there is a monster out there which he has stitched together and brought to life, he also must take responsibility for the monster's actions. Victor can't bring himself to do that, so he allows the innocent Justine to take the blame and be executed instead.

This decision has a snowball effect, and Victor is buried in an avalanche of lies and deception about this being he has created. In the end, his inability to tell the truth will cost him the lives of everyone he holds dear.

Frankenstein doesn't want anyone else to know about his hideous creation because he's racked with guilt over the great evil he's unleashed upon an unsuspecting world. If he starts telling other people that he, the man that they know, love, and admire, is responsible for this utter catastrophe, they will, at the very least, think less of him.

It's rather telling that Frankenstein chooses to tell his story, not to his friends and family, but to a complete stranger, Robert Walton. It's almost as if he's telling a story about someone else, a story about a crazed scientist who, drunk on his own genius, created a vicious monster that quickly got out of control. Telling the tale this way allows Victor to distance himself from his own actions while at the same time being completely frank about the responsibility he bears and the guilt he fears for everything that's happened.

In Chapter 5 of Frankenstein horrified by what he has created, Victor Frankenstein rushes from the room and stays all night in the courtyard until he is certain that the hideous creature he has created is gone. Victor, who relates his history to Robert Walton, his rescuer, tells him,

How can I describe my emotions at this catastrophe, or how delineate the wretch whom with such infinite pains and care I had endeavoured to form?....breathless horror and disgust filled my heart.  Unable to endure the aspect of the being I had created, I rushed out of the room.

Having been frightened himself at his creation, how could Victor Frankenstein tell others without their considering him a madman or being so terrorized themselves: 

I dreaded to behold this monster; but I feared still more that Henry should see him.

After Victor joins Henry, he becomes wild and nervous; in fact, he falls ill.  Ashamed of his work, abased by the outcome, Victor's pride tries to cover his fears as he is with Henry.  For, always he has been known as a most intelligent youth and young man. To suffer the explanation of his hideous creature is more than the pride of Victor Frankenstein will allow.

Later on, in Chapter 22 Victor confesses that he avoided explaining his claims of responsibility for the death of family members and friends:

I avoided explanation, and maintained a continual silence concerning the wretch I had created.  I had a persuasion that I should be supposed mad; and this in itself would for ever have chained my tongue.  But, besides, I could not bring myself to disclose a secret which would fill my hearer with consternation, and make fear and unnatural horror the inmates of his breast.  I checked, therefore, my impatient thirst for sympathy and was silent when I would have given the world to have confided the fatal secret.