You may not intend to behave in a critical way toward your friend, but over time, that's the habit you'll develop if you aren't careful. When they try to express their ideas, you'll pick at their logic and expose the flaws in their thinking. If they tend to be indecisive, you'll push them to choose a practical course of action based on the alternatives available.
But to them, you'll come across as meddling, critical and controlling. Your efforts to help them focus and define their thinking will feel restrictive, as if they aren't allowed to think in the way that comes most naturally to them. Worse, they might start to believe you don't recognize or appreciate their intelligence. Let your friend think and express themselves naturally -- and allow them to make mistakes. If you can do that, then this dynamic will actually have a positive outcome -- that of teaching your buddy to think more carefully and thoroughly, and you to be less critical of others' ideas and self-expression.