Great answers above, and mine will not add much to the conversation but I'm saying it anyway.
At our relatively small high school, an experienced runner would fairly easily make the varsity XC team, but once on the team he'd have to realize that the goal is to win the state championship. They work extremely hard, pushing every guy on the team to excel. It's a rough environment with a rough-edged coach who wants 100% commitment from every team member. They workout together every day in summer, before school, after school, and attend meets several times a week in the fall, then workout the rest of the school year "unofficially." To make captain, you'd have to be the team's best runner, passionate about winning state, eager to please the coach, an effective motivator for team members, and somewhat of a public face for the team at pep rallies to get the rest of the school to care about winning state almost as much as the team wants it.
So, I don't know, call me crazy, but the whole notion of signing up for the XC team junior year or senior year, and raising your hand when somebody says "Who wants to be captain?" -- well, that just would not ever happen here. Captain of the varsity XC team is going to be somebody who has devotedly run XC since 6th grade or who comes on quick and strong in high school and exhibits extreme leadership ability.
I guess I get annoyed by so many posts in this forum that make it sound like being captain is as easy as just showing up.
That said, I realize that at many schools, "captain" is a revolving door and doesn't mean much. I'm happy to say that's not the case here.
Same goes for the golf team, by the way. Highly competitive team, though I'd venture that the workouts are not nearly as extreme physically. Team captain is usually someone who ranks in the top 20 high school golfers in the state. You're not just going to walk onto that team and make captain.
To the OP: Passion is a virtue. Passion for a sport means you want to excel for others just as much or more than wanting it for yourself.