In Shakespeare's Macbeth, the character Macbeth is pliable to external thoughts, specifically external thoughts from the witches and Lady Macbeth. As is typical in Shakespeare, however, the reasons for this are ambiguous. This ambiguity is one of Shakespeare's attributes that make him so popularwith readers/viewers, as well as actors. Much is left up to interpretation. While some evidence exists that Macbeth is weak in character, there are certainly other possibilities. Is it possible to be a weak person and take such as active part in battle? He is not only a soldier, but a leader in battle. Once the deed is done (Duncan killed), he acts decisively (though foolishly) for much of the remainder of the play. In fact, acting without external advice brings about his downfall (arranging the killing of Banquo without seeking the advice of Lady Macbeth, for instance).
A better possibility concerning why Macbeth is so pliable to external ideas is that he possesses relentless ambition. This is his tragic flaw. He has scruples that his wife is afraid will get in the way of killing Duncan. He has second thoughts about killing Duncan, because Duncan has treated him well and has been a humble monarch. Yet, his ambition is such that with a little nudge he will abandon what he knows is just and do whatever is necessary to become king. Once he is crowned, he will do whatever is necessary to stay king. His ambition overcomes his scruples, with just a little help from others.
Most people say that Macbeth is vulnerable to going along with what other people tell him because he is essentially a weak person. He has great ambition, but at heart, he is not a very strong man.
We can see this at various places in the play. It is only when the witches make their prophecy that he thinks of killing Duncan. But he cannot really bring himself to do it until his wife urges him to do it. Even then, he is not really commited to it -- before he does it, he has the vision of the dagger. After he does it, he feels so guilty that he cannot pray.
So the major reason here is that he just does not have a very strong character.