Just as "freedom of speech" is over rated, "sense of humor" is over rated.
Everyone will think differently. What you and everyone else may think is funny or humorous, most of the time, it is at the expense of someone else. If that person did not see it as funny, the humor falls apart.
Sure, everyone can lighten up, but feelings do not go away that easily. If you are hurt, wounds take time to mend and heal. Unless you're some odd mutant with extremely rapid healing powers, don't tell those wounded to lighten up. Understand their feelings too.