[Formendacil]

Originally Posted by Galadriel55

Aiwendil has an interesting explanation for that - that accepting human form forces you to accept human weaknesses and limitations, if I understood it correctly. However, I think that the human form only affected the physical (more or less) part of him, and not the mental (or psychological). Taking a form is like putting on clothes for the Ainur, and clothes don't usually affect the way you think and feel. Of course, this is up to speculation, and I guess it depends on your personal opinion.

By coincidence, I was just reading a portion of Tolkien's Letters that addresses Gandalf and his humanity/lack of humanity. Since I know where to find it easily, let me quote:

Originally Posted by Tolkien, Letter 156, to Robert Murray SJ

But G[andalf] is not, of course, a human being (Man or Hobbit). There are naturally no precise modern terms to say what he was. I w[ould] venture to say that he was an incarnate 'angel'– strictly an ἄγγελος: that is, with the other Istari, wizards, 'those who know', an emissary from the Lords of the West, sent to Middle-earth, as the great crisis of Sauron loomed on the horizon. By 'incarnate' I mean they were embodied in physical bodies capable of pain, and weariness, and of afflicting the spirit with physical fear, and of being 'killed', though supported by the angelic spirit they might endure long, and only show slowly the wearing of care and labour.

--emphasis mine

...and, from the same letter, regarding Gandalf the White:

Originally Posted by Tolkien

Gandalf may be enhanced in power (that is, under the forms of this fable, in sanctity), but if still embodied he must still suffer care and anxiety, and the needs of flesh.

Actually, the letter is worth reading in its entirety, but that's a lot of copying... In any case, I think the point should be sufficiently clear that the incarnation of the Istari was not like putting on clothes--it was not merely assuming a form, or wearing it; rather, it was becoming it.