I got pink eye about a month ago, got drops for it and it went away. Since then I've gotten it two more times, why does it keep coming back?

Environment. The germs may be spread on surfaces like phones, doorknobs, etc. See your doctor to make sure it's pink eye. Sometimes allergic eyes can look like they are infected, too. Keep hands washed or instant sanitized, try not to touch your eyes.
See eye doc. Pink eye (conjunctivitis) may be due to viral, bacterial infection, environmental/allergic, or inflammatory causes. Wash hands, don't share towels, disinfect doorknobs, kitchen, bathroom surfaces; check if you have new brand of eye makeup (may be allergic); if old, may be contaminated. May be inflammation from eye or unrelated organ (e.g. Inflammatory bowel disease). See your ohthalmologist.
Pink eye. Pink eye, or conjuctivitis, can be due to many things, including viral or bacterial infections as well as allergy. Allergic conjunctivitis tend to be recurrent, and are chronic (vernal). Infectious conjunctivitis can be recurrent if you live or work in a crowded situation or wear contacts, which can perpetuate the infection. Wash your hands often, avoid rubbing your eyes, and see a doc for eval.
Contact lenses? If you wear contracts, you need to throw out the old pair, as they can harbor the infection. If not, then see your eye doctor.