Protecting His Friend, or Himself?

     Have you ever had one of those days where someone said one thing, and it ruined your entire day, and leeched into the next day? That was today. I was chatting with a few friends last night, at dinner, when I mentioned that a mutual friend was interesting and I'd like to get to know him better. Nearly everyone at the table was okay with this decision, but the friend who had introduced us (we'll call him Jack) completely put his foot down. The others: (and these are not their real names) Edward, Lucy, Collin, Peter, and Bernard actually seemed supportive, but Jack insisted that our mutual friend (Calvin, for lack of his literary favorite) was interested in someone else, and this is after Jack'd tried to pair him with a girl from class.
     Normally that'd be okay, but the way he phrased it as "Calvin is off-limits" rather than just suggesting I wait until he'd figured out what's going on with that to pursue anything new. It made me angry that Jack was butting into my life, and kind of indignant that he was butting into Calvin's life. We're at completely different points where we want different things from our relationships and Jack doesn't seem to get that I'm going to do my best not to hurt Calvin.
     I keep trying to figure out why he'd do something so decisive as step in front of any "relationship train" that might move, and a couple of my friends and I came up with answers that don't fit with what I know about Jack: he likes me (that's not going anywhere and he's the kind of guy to move on when he realizes it's not), he's afraid I'll hurt Calvin (okay...), Calvin will hurt me and he wants to prevent that (really?!), he's afraid to lose Calvin to a relationship like he lost Peter to a relationship (that's not going to happen any time soon), and there are more, but those are the ones we're dealing with now. I just want Jack to let whatever is or isn't going to happen happen, let us figure this out for ourselves and you'll keep both friendships and might even develop both further. The only thing that's made the day any better is my other friends' encouragement, that's what friends are for: to make your day better.