Want to be a writer? Prepare for pain. The pain of sleep-deprivation, the pain of rejection, the pain of carpal tunnel syndrome, the pain of a good face-keyboard smack when things just aren't working ... all of this and more is likely in your future.
That's not entirely why I think a touch of masochism is a prerequisite, though. Those things all come with the package, and we have to find ways to deal with them--like power naps and ergonomic office furniture. The masochism comes in with the pain we (should) intentionally seek: the sting of constructive criticism.
Personally, I love getting feedback specifying certain aspects that aren't working for the reader, but that sting still pricks me now and then. Still, I'd rather endure that minor pain than get a inbox-full of, "This is amazing and should be published right now!" While the latter is nice for the ego, it doesn't actually help me improve, and even if I got a publishing contract tomorrow, I would always have room to grow.
A parallel: In my day-job, an administrator observes my class a couple of times a year for evaluation. I've yet to have an administrator with a math teaching background, so the fact I can teach calculus already impresses them. More often than not, the feedback is something like, "You're doing great--keep it up!" Once in a while they remark on a small item they can tell was more because they were in the room than anything else. (My fingerspelling skill takes a nosedive when other adults are in the room ... definitely gotta work on that.)
I know I'm a good math teacher, but I also know I'm not perfect. I can identify certain areas for improvement on my own, but for others, I could really use an outside observer to tell me if something works or not, or if I'm doing things I'm not aware of.
Same thing with writing. If a reader isn't feeling my MC's emotion in a certain scene, I need to know. If a particular section is boring, I need to know. When those are areas I've worked on and think are great, finding out they might not work that well can hurt. The biggest hurt is when someone clearly doesn't understand my intention. Those are the moments I doubt myself, wondering if I have any idea what I'm doing, assuming my own failings led the reader to misconstrue the concept. But I will still seek out those opinions, weigh them against each other and against my own instincts, and try to incorporate what I learn into making my writing better.
Turning it around, then, writers should not be sadists. When we're offering critique, it's important to be honest--as noted above, glossing things over won't help anyone--but not intentionally cruel or derogatory. Telling someone, "This sucks--you're never going to make it," is no more helpful than gushing why-isn't-this-published-yet praise.
Most importantly, we have to make sure we aren't such masochists that we lock ourselves into the editing/revising phase for eternity. At some point, you have to decide that it's good enough to get out there and submit ... and ready yourself for those darts of rejection.