Why does Macbeth put so much faith in the witches in Shakespeare's Macbeth?

Well, first he does, then he doesn't, then he does. Sort of like a fish nibbling at the bait...

When Macbeth first meets the witches, he is overcome by the possibilities presented to him (Act 1, scene 3):

MACBETH:

[Aside.] Two truths are told,

As happy prologues to the swelling act

Of the imperial theme!—I thank you, gentlemen.

[Aside.] This supernatural soliciting

Cannot be ill, cannot be good. If ill,

Why hath it given me earnest of success,

Commencing in a truth? I am Thane of Cawdor.

If good, why do I yield to that suggestion

Whose horrid image doth unfix my hair

And make my seated heart knock at my ribs,

Against the use of nature? Present fears

Are less than horrible imaginings:

My thought, whose murder yet is but fantastical,

Shakes so my single state of man that function

Is smother'd in surmise, and nothing is

But what is not.

Yes, he is moved to believe in them and the future that they offer him and is already thinking of murder. Then, quickly, he has second thoughts and decides maybe things will happen the way they happen; he'll just wait and see:

MACBETH:

[Aside.] If chance will have me king, why, chance may crown me

Without my stir.

Along those same lines, later in scene 7, Macbeth figures he has it good enough, he's still the captain of his fate, and the heck with what the witches say; he'll just keep things as they are:

MACBETH:

If it were done when ’tis done, then ’twere well

It were done quickly. If the assassination

Could trammel up the consequence, and catch,

With his surcease, success; that but this blow

Might be the be-all and the end-all here,

But here, upon this bank and shoal of time,

We'd jump the life to come. But in these cases

We still have judgement here, that we but teach

Bloody instructions, which being taught return

To plague the inventor. This even-handed justice

Commends the ingredients of our poison'd chalice

To our own lips. He's here in double trust:

First, as I am his kinsman and his subject,

Strong both against the deed; then, as his host,

Who should against his murderer shut the door,

Not bear the knife myself. Besides, this Duncan

Hath borne his faculties so meek, hath been

So clear in his great office, that his virtues

Will plead like angels trumpet-tongued against

The deep damnation of his taking-off,

And pity, like a naked new-born babe,

Striding the blast, or heaven's cherubin horsed

Upon the sightless couriers of the air,

Shall blow the horrid deed in every eye,

That tears shall drown the wind. I have no spur

To prick the sides of my intent, but only

Vaulting ambition, which o'erleaps

And falls on the other—

Then in comes Lady Macbeth and she chastises him with her tongue and convinces him to believe again in the witches' prophecy and promise of greatness. Finally, he agrees:

MACBETH:

Bring forth men-children only,

For thy undaunted mettle should compose

Nothing but males. Will it not be received,

When we have mark'd with blood those sleepy two

Of his own chamber, and used their very daggers,

That they have done't?

So, he's back again and says, at the end of the act:

I am settled, and bend up

Each corporal agent to this terrible feat.

Away, and mock the time with fairest show:

False face must hide what the false heart doth know.

Now he's caught.