See, then you'll understand why they're lying. It's not always bad -- the people lying about their meat consumption figure it still helps to promote the cause, even if they themselves can't live up to it. Christians do the same. The asshole at work who rages at you when you offer criticism? Watch carefully -- more often than not, they'll quietly change their habits in response to what you said. The "How dare you question me!" bullshit is just insecure posturing, and that's never the part that matters.
Then, to take this from the other side ...
I guess this just means "Be honest," and no, my counter isn't "Lying can fix almost any problem!" (Even though that happens to be true.) No, my better advice is,
Learn to read the goddamned room, people.
It doesn't matter what you say. At all. What matters is what the other person hears. People aren't robots, and all words are filtered through a thick layer of mood and context. If someone's convinced you're out to get them, then literally any criticism, no matter how logical and carefully explained, will be heard as "You suck, I rule, I am establishing my dominance over you." If the other person thinks you're incredibly attractive, the only information they'll retain from your exchange is "My crush noticed me!"
If you speak honestly, openly, and clearly and the other person reacts badly because you didn't take five seconds to first consider their state of mind, that's on you.