Why? Because I keep forgetting to bring my cloth bags into the grocery store! They are right in the back of my car all the time. But I don't see them , so I forget. I even say out loud, "don't forget the bags" as I'm driving in to the store parking lot...uh, I have no attention span, so no bags! I even have a note on my steering wheel that says, "Bags" Does that help me remember? NO! Not until I'm in the check out lane do I remember my lonely bags out in the car wanting to do their part but unable to because I'm a putz. I can feel their disappointment when I open my hatch and I swear I can hear the plastic bags mocking them as I unload my goods.
Does this really mean I'm singlehandedly ruining the planet? Probably not. I treat the planet pretty good other than this one aspect-I recycle cans and plastic, return my plastic bags to the store for recycling, use old newspapers for shipping material and re-use every box I've ever gotten for shipping, storage, costumes, projects...you name it.
Will I continue to try and remember my cloth bags? Yes and hopefully it will become a habit for me. Until then, will I lose any sleep while "ruining the planet"? Umm...no.