Jesus said that we shall fast (Mk 2:20. Mt 9:15) and fasted himself (Mt 4:2). St. Paul directed us to "exhibit ourselves as the ministers of God" by a variety of mortifications, including fasting (1 Cor. 6:4f). So, Catholic Christians fast.
It doesn’t say, “if your brother demands that you fast and you don’t think you should have to”
Well, what are you doing on this thread then? Your fellow Christians fast and urge others to fast and you apparently take offense. We have a dispute. We should ask the Church, which shall "bind and loose" in all matters (Mt 18:18). I have.
I am not the least bit offended. I just have questions, and I can’t get a clear answer.
You know that the two passages you cited in Mark and Matthew are the retelling of a parable Jesus told. It was about having and not having Jesus here on earth. Having Him here is the feasting, not having Him is the “fasting”. I know you’ve been told otherwise, but can’t you see it for yourself?
Not one place in scripture are we commanded to fast, although it can be a very good and useful practice. But it must be willingly undertaken or it is empty.
1 Cor 6 is another passage about resolving disputes among believers. It says we are supposed to resolve them within the community, not take it to a secular court of law.
Nothing there about “mortification” or fasting.