In everyday sociality, people don’t usually think things through before they say what they want.
In most cases, it turns out alright, but on the off-chance that it doesn’t, the effects are not always visible.
If, for instance, you have a proclivity to tease other people and to bring them down through your sense of humour, you would not necessarily have any way to tell, or incentive to know, whether the other person took it badly or not.
The relation between facial expressions and affect (our thoughts, feelings and attitudes) has been well-established in the research literature, but you can’t know for sure whether the other person is going to be that expressive; maybe s/he is aloof, for all you know.
Therefore, one would be well-advised to treat others benignly with regard to one’s ignorance of the other person’s life experiences and background, what I’d like to term alterous sympathy.
You have no idea who you’re talking to, whether the other person has been going through domestic abuse, have had family problems, been raped, bullied or the like among the myriad causes of deterministic misery.
These pains may become persistent in certain people’s minds for several years, and to treat them harshly and insensitively for no reason, by expecting them to conventionally be on the same arbitrary playing field as everyone else, amounts to the perpetuation of an individual’s complex set of interrelated pathologies.
What you say ultimately matters: if the other person acts withdrawn or reserved, include them wholeheartedly or let them be.
If you find yourself in the same group together, give them a break and at least don’t come with sneering comments or mockful humour their way; that’s going to manifest itself as cruel and inhumane from the other person’s perspective, even if it wasn’t deliberate.
One thing’s for sure: those scars won’t fade away quickly, just as the disparaging comments we receive on a regular basis hurt us more than we’d like to admit.