Why does lady macbeth fear about her husband after she reads his letter

Lady Macbeth is initially very excited upon reading the letter. It seems that the witches have put a supernatural seal of approval upon her ambitious plans for Macbeth. Greatness is not just within their grasp; it has been promised to them by the forces of darkness.
At the same time, however, Lady Macbeth is somewhat apprehensive. She's absolutely determined in her own mind to do whatever's necessary to put her husband on the throne. But the same can't be said of Macbeth himself. Prophecy or not, he still has to take the fateful step to make it happen, and Lady Macbeth's not entirely sure that her husband has the guts or the outright ruthlessness to kill Duncan and take his throne:

Yet do I fear thy nature; / It is too full o' th' milk of human kindness / To catch the nearest way: thou wouldst be great, / Art not without ambition, but without / The illness should attend it. (Act I, Scene v).

Lady Macbeth readily concedes that her husband has the necessary ambition. But does he have the requisite mean streak within him? She's not so sure. Still, once Macbeth has returned home, his scheming wife is certain that she'll soon talk him round:

Hie thee hither, / That I may pour my spirits in thine ear / And chastise with the valor of my tongue / All that impedes thee from the golden round, / Which fate and metaphysical aid doth seem / To have thee crowned withal. (Act I, Scene v).

Both fate and witchcraft have determined that it is Macbeth's destiny to be king. And Lady Macbeth's not about to let her husband miss this once-in-a-lifetime opportunity.

You will find your answer in Act 1 Scene 5 of the play.
After reading the letter, Lady Macbeth struggles with deep concerns. She fears that her husband won't have the courage to act purposefully. She doubts that he will pursue the kind of ruthless action needed to facilitate his kingly ambitions. Essentially, Lady Macbeth is afraid that Macbeth lacks the necessary aggression to pursue his goals, especially if the actions called for require him to renounce his moral beliefs.
She believes that Macbeth is "too full o' th' milk of human kindness/ To catch the nearest way." Lady Macbeth readily admits that Macbeth is "not without ambition." What is lacking, however, is the will to act without fear of the consequences. So, Lady Macbeth hopes that Macbeth will hurry home. Then, she will be able to wield her influence over him and motivate him to act to secure his position.