[wpietri]

One good reason: ignorance.

If I were to say "lighten up", it would basically be asserting that I am a better judge of the impact of a comment than the person who's upset. But as relatively privileged person, I am much less often on the receiving end of offensive comments, and I have more resources to deal with situations like that.

If I were to say something racially tinged to a black person and they thought it offensive, I would be strongly inclined to credit them because they've had a lot more opportunity and a lot more incentive to think about the nature of racism.

I agree it's not a perfect rule, but it's certainly the way to bet.

Another big reason to do so is power. Things like "lighten up" are used by privileged groups to suppress criticism because that helps maintains the power imbalance. Not consciously, mind you, but as apes we're hardwired for status.