You could also always be hungry because you're not giving your body nutritious, healthy, satisfying food. Try eating somethings like yogurt, peanut butter, (items with a lot of protein) also fruits and vegetables. This will hopefully give you a fully meal and leave you satisfied so you're not hungry 20 minutes later.
This could be a result of many things. This could be because you could be pregnant(like stepahniej said), stress, boredom, body chemistry, and believe it or not, the people you hang out with(You tend to aquire the same behavior as those around you).