There are two types of people: the people who are responsible enough to care for plants, and the people who are not.
Your girlfriend falls into the first category. She is the type who will run through the house with crazy eyes, yelling at you while you're playing Tetris on your iPad, saying, "We need to clean the bathroom, mop the floors, and dust the baseboards!!!!!!"
Your attitude is more along the lines of, "Well, we don't need to do anything."
Her version of being an adult is being responsible, and your version of being an adult is not having to answer to anyone. YOUR MOM ISN'T HERE. YOU DON'T HAVE TO MOP THE FLOOR TONIGHT IF YOU DON'T WANT TO.
This may seem lazy, but it's smart to isolate tasks and responsibilities to the shit that really matters in order to preserve your energy and balance out your stress levels. For example: Feed the cat? Yeah, that needs to happen every day. Water the plant? Eh, who cares.
What have we learned here? You'd be terrible at taking care of plants (or nurturing any kind of life, really), but your girlfriend's control freak tendencies and inability to relax would make her the BEST at it. Buy her this Ceramic Pig Planter ($29.99, Fruit Fly Pie) and watch her happily add yet another task to her already long to-do list.