Maybe it's because it's only been recently that I stopped thinking that my first name was the ugliest sounding and looking name in the entire world, but I have never uttered anything along the lines of, "I'm just being Megan" or "but that's just me. That's Megan, and if you can't let me be Megan, then blah blah blah."
Whenever someone has done someting particularly selfish, problematic to someone else, self-centered or just plain rude, the excuse is generally that being selfish, problematic, self-centered and rude is part of being this person's first name. Any request by an outside party for this person to be considerate, easy-going, or polite is interpreted as an attempt to keep this person from epitomizing their first name.
Yelled too much at someone else for no good reason? "I'm just being Megan. And if you don't want to hear Megan yelling for no good reason, you should just get lost because unless you want to sit around and be yelled at for no good reason, you're stifilng Megan from expressing her loud no-good-reason to be yelling yelling."
Left dirty rotten food-covered dishes in the sink? "Don't you call me a slob. I'm Megan. Megan is no slob, Megan just leaves filth around."
Rude to someone on the phone? "Megan doesn't answer the phone for other people. Megan answers the phone for Megan, and if you have a problem with it, answer the phone yourself."
Again, maybe I'm crazy, but when I am exhibiting negative characteristics, the last thing I want to do is permanently associate those things with my being, my name and my personality. If rudeness, sloth, filth, and selfishness are as inseperable from me as my birth-given first name...well, you get the point.