They were thoughtless and ignorant.
Most of the people praised for their "positive outlook" are just putting on a front in public. Not all, but most.
You can feel for Mark, but it's irrelevant to what you're experiencing. Just like what you're experiencing is irrelevant to him. The fact that somebody else is suffering in a different way doesn't alter your suffering, doesn't make it easier to bear, and is not relevant to what you need to do to manage your own condition.
I hate people who put suffering into this bullshit hierarchy and use other people to make themselves feel better or fill people who are already suffering with guilt for daring to suffer when "it could be worse".
If you take it to its logical conclusion, then only one living person on the planet would be allowed to be mentally suffering at any one time because they would be the one person at the top of the hierarchy of suffering to whom nobody could say "ah, but so-and-so has it worse". Clearly ridiculous.
Likewise, you wouldn't tell somebody grieving for a loved one that they're not allowed to grieve because Jane next door just lost two loved ones. Both are sad. Both are valid.
If A stubbed their toe one assumes it would still hurt and they'd still react to that pain. The fact that Mark has had a leg amputated wouldn't stop A's toe from experiencing pain. We're individuals living separate lives.
As for "you have to feel grateful"... No, you don't. You don't have to feel anything.
Yeh, maybe just don't talk to A anymore. Is that possible?!