Few things satisfy as much as giving advice and having it taken to heart.
You feel wise and the one who benefitted believes he’s been granted good
counsel. Sometimes, though, we’d rather feel in command or victorious than
wise; so we might offer self-serving advice and wind up deceiving rather
The Torah considers that to be on par with “set(ting) a stumbling block
before a blind person” (Leviticus 19:14), i.e., it’s like “placing a
figurative 'stumbling-block' before someone 'blind' to anything” as
Ramchal explains it. That’s to say, it’s like tripping up someone who
trusts you and couldn’t imagine you’d fool him like that.
What we’re expected to do is to “pass on the clear and unadulterated truth
to whoever might come to you for it”, regardless of “whether you’re going
to benefit by the outcome or not” Ramchal explains.
That’s difficult, to be sure, which is why “so many people stumble when it
comes to this every day” Ramchal admits. Because many of us have
too “strong (an) urge for profit” and prestige. But “an honest person’s
duty when someone comes to him for advice” he submits “is to offer the
self-same advice that he’d give himself, and for no other reason than for
the good of the person asking for it.”
There’s an exception to that, though. If the intentions of the person
asking you for advice are to do harm somehow with the advice you’d offer,
then it’s “certainly a mitzvah to deceive him. As it’s said, ‘With the
perverse you act perversely’ (Psalms 18:27)”.