Wow - this is something I have thought a lot about, but never really talked (or typed) about out loud.
I was able to be friends (and even roommates) with other SM's in college, because our professors pigeon-holed us into different arenas, so we were never directly in competition for shows. I was always stuck with either the One-Acts series or straight plays, while my roommate Libby got the musicals and my friend Jenaiah got dance. (Funny how I ended up SM'ing ballet and Jenaiah is in Shakespeare.) We still keep in touch, mostly through facebook, and once again, there is no competition because we all live in different states.
When I started working in professional theater, I was eager to make friends with as many SM's as possible, due to making strong contacts and learning as much as I could about their personal SM styles. I've never felt in direct competition with another SM friend because 1) I'm generally happy where I am and have a season-long gig, and 2) These friends have helped toss me work in my off-time. I actually landed my resident SM gig 7 years ago because a friend SM recommended me to a PSM that needed an ASM. And here I am! The PSM has moved on and I've moved up.
I've made a point to always provide work or pass along work to other SM's in my area, whether they are union or non, depending on the job. I always appreciate it when my fellow SM's give my name for a job when they aren't available for it themselves. I think this constant "passing the job along" keeps the jealousy down and keeps friendships healthy. I suppose I would feel more competitive with other SM's if I were trying harder to get summer shows. But I haven't, so it makes me rather neutral on the subject.
As far as career envy goes - sometimes I wish I could get back into certain theaters and when I have friends working in those theaters, I do get a bit envious. Though, 98% of the time, I wouldn't be able to work in those theaters due to my own schedule conflicts - so I really have nothing to be envious about. I'm happy that my fellow SM's have work, and that I have work, and that we're always looking out for each other. Whether it's referring another for a job, giving someone a job, or banding together to work on a project (like a Workshop festival).
And I'm thankful for the SM's I've met (electronically) on here, that have helped me with my issues and concerns, and that have offered excellent advice. I've only been in the real world for 8 years, so I still have a lot to learn and experience. If it weren't for SM friends, I think I might feel a bit lost.