I have a very painful boil between my butt cheeks. i get them quite often. sometimes they are small and sometimes huge. why do i get them?

Several things...: ...could do this. Maybe you're colonized with staph, strep or other bacteria causing infection. Continued moisture from sweat can contribute, especially if you are overweight. Diabetes increases skin infection risk. Pilonidal sinus or perianal fistula if the problem is always at the same spot. Hidradenitis suppurativa, a chronic inflammatory problem. See your doctor or maybe a dermatologist.