According to Hosfstader, Lakoff and others, metaphor is the core of cognition, the basic algorithm upon which all else runs. It is deeply embedded in the way we perceive the world, and this is reflected in our language; Lakoff catalogued some of the "metaphors we live by" in his book of the same name, and it is surprising how transparently we use metaphor all the time without even being aware of it.
As an intuition pump, consider the following vaguely associated words and their antonyms:
up / good / positive / forwards / fast / white / on / daytime / hot / red / aroused / friendly
down / bad / negative / backwards / slow / black / off / nighttime / cold / blue / bored / indifferent
I'm sure you can add more. You can pick almost any pair of these words and find a phrase that relates them - "I'm feeling blue because Claire called off our date; she's been cold ever since Dave bad-mouthed her."
Interesting, that book caused me to question what I really know and hold my views on metaphore. When you see slime mold "learning" and decent AI emerging, it really starts to throw a wet towel on how amazing we think humans are in general because of our intelligence. Then there's election night.
Sorry, I just read your comment. That teacher just said it like "very often", not that is like a rule that every comparison is a bad argument, but more often than not we tend to go the way of comparisons (think about whataboutism for example)
Comparing a new thing to an old familiar thing or a current fact can really help people overcome irrational bias against the fear of the unknown.
Metaphors, similes, and analogies are all comparisons and all recognized as powerful forms of rhetoric.