It's probably because you often let it get down until it has to yell at you to plug it in. While it's not good to always leave your phone plugged in, once it is charged, letting it get down to red is literally destroying the battery. Over discharge of the battery ruins it. As soon as it tells you to plug it in, do so. Your battery is literally eating itself because it is so hungry. Don't use it once it starts to get low, unless it's an emergency, and get yourself an external battery pack for emergencies. (Or for when you're really bored and wanna watch a movie and can't find an outlet.)