DISCLAIMER: still not a reference to anyone I know now.
Eventually, after having to smack enough chronic line-crossers in the face with shovels, I did figure out that I do not owe it to anyone to be that nice and accommodating. Allowing people to cross your lines, however, is only part of the problem. Another part is figuring out where your lines are and which ones you really care about if they're crossed.
When I was younger, I had a plethora of lines and - now to my regret - I was inconsistent in defending them. I could blame it on bad days, wavering self-esteem, changing ideals, what have you, but ultimately it was just that I didn't have very concrete lines or consistent consequences.