While sometimes it is just pure genetics, there are a few different factors that can contribute to oily skin on the face and body. Some of the most common contributors to oily skin include things such as climate/environment, diet, hormones, age, stress, and product usage.
Oily skin is often associated with large pores. These two go together because large pores allow for extra sebum production, leading to oily skin. Unfortunately, having large pores makes you more susceptible to acne and blackheads. This is due to the fact that large pores become clogged very easily by things like dirt, makeup, and yes, oil.
Identifying your personal skin type is the first step in properly caring for your skin, as well as a determining factor in which types of products are best for you. Keep in mind that skin type can change not only over time, but also day to day. Fluctuations in hormones, climate, and diet can all contribute to skin type (think things like your menstrual cycle, vacation climates, eating foods high in sugar or salt).