There are many kinds of eczema, the most common ones are Atopic Dermatitis (caused by genetics). And Contact Dermatitis (caused by touching an allergen or something your skin is sensitive to).
Atopic Dermatitis. It's caused by genetics. Babies with eczema usually have family members with a history of asthma or allergies. It's actually common in babies. 1 in 8 babies in the US has it. In places where pollution or weather is harsher, the statistics could be higher.
Contact Dermatitis. It's caused by touching things you shouldn't, whether you're allergic or sensitive to it. It could be triggered by many things, from clothes to sweat to fragrance to food to air pollution.