Hygiene. Gloves are changed to protect you from contamination of other patients and the doctor/assistant.
? I'm not a dentist but I do surgical procedures and usually change gloves for two reasons: 1) I've punctured it or think I have; 2) I've contaminated it and need a new sterile one.
Sterility. Gloves are changed to maintain the chain of sterility. If something possibly non-sterile is touched, such as a light switch on the wall, or a drawer-pull, gloves will be changed. If glove is punctured and no longer acts as a barrier, gloves will be changed. If messy materials, such as impression material, coats gloves, they will be changed. Glove change is done for safety and comfort.
Steralization Chain. Dentist and orthodotist will change gloves instead of re scrubbing when they feel the chain of steralization is broken. Perhaps from touching a non sterile surface like a light swiitch or a dropped instrument.