What Causes Cysts To Reoccur In The Ovary?

I am 30 years old. My weight is 45.& height is 152cm. I had cyst(5cm) in my left overy 8 months ago and removed it by open operation.
After one month scanned and also seen small small cyst. Then Doctor advised to take injection. I took Three injection in 3 months.
(1 injection cost =Rs.9500/-). After two months I scanned again and
see another cyst in the right overy. Its size is 3.2cm. Why this come again. I have thyroid also.