Went to a dermatologist. he found 2 dark moles. he said i should keep an eye on them. what does that mean? should i worry? why didn't he remove it?

Good idea: to observe , few pigmented moles transform to malignant type the serious melanoma , where size of mole increases , intensity of pigmentation changes ,some tomes will have ring like pigmentation with a hallo , then will need excision biopsy. Second choice simple removal histological analysis (biopsy ) then you don't have to worry. Speak to your doctor and you want it out.