Why does Macbeth seek out the witches?

To answer this question, take a look at the events of act 3 scene 4. At the beginning of this scene, Macbeth learns that his henchmen have killed Banquo but Fleance managed to escape. Later, at the banquet, Macbeth sees the ghost of Banquo and Lady Macbeth becomes very worried about her husband's state of mind. In addition, Macduff does not attend the banquet, leaving Macbeth to wonder why he does not come when commanded.
As a result of these events, Macbeth tells his wife that he will go and visit the witches the next day. He says to her:

For now I am bent to know,
By the worst means, the worst.

In other words, Macbeth wants to know what all of these things mean and, more importantly, he wants to know what he can expect in his future. Will things get worse for him or is his kingship safe?
Macbeth is, therefore, becoming increasingly paranoid about his position as king and feels that the witches are the only ones who can allay his concerns.