Why is it that I always get a cut on the side of my mouth?

Angular cheilosis. If you are talking about the area where your upper and lower lips meet at the corner of your mouth, that is known as angulae cheilosis. It a pooling of saliva which the tissue to break open due tio constant trapping of saliva. It is sometimes aggravated by vitamin deficiencies. It can often be ressolved by applying vitamin E oil repeatedly to the area. Sometimes topical antibiotics are necessary.