To be nice, or to be strong?
People instinctively view that as the choice they're forced to make. I've been there before, and I've seen others who view it the same way. I've caught myself thinking (and heard other people saying) that:
"I'd love to be a nice person, but if I'm going to be walked all over for it, I'd rather be tough and not have as many people like me."
But do we really have to pick only one of those options? Do we have to come across as douchebags and jerks in order to keep people from taking advantage of our generosity? I don't think so. A person can be very kind to their friends only (for instance, because I trust mine not to take advantage, I will do things for them even before they ask). It's a different story, though, when it comes to coworkers or strangers. I've learned through countless of my own experiences that I can't afford to give strangers or mere acquaintances that much leeway because they so often abuse it.
Do I come across as a jerk? Hmm...probably, at times. But I think that some people do make judgments between "being a jerk" and "looking out for yourself". And for those who don't? If I'm not close enough to trust them, I'm not close enough to care how they view me, and that's pretty much all there is to say.