For some people it is just the way their metabolism works. However sometimes it is due to an overactive thyroid. If you have not had your thyroid profile done, then do so. Also have CBC, Calcium and Vitamin D levels checked.
Otherwise focus on making sure you are consuming healthy foods, increase your consumption of dried fruit and nuts, and consider lifting weights. If it is the way you have always been, and you are otherwise healthy (active, energy to do the things you want or need, able to focus on work or studieals, no chronic health problems, etc.) then consider yourself lucky that you don't have the opposite problem.