You seem offended that they assumed it was OK to put your personal belongings in your room. Please understand they they felt equally offended that you assumed it is OK to leave personal belongings in the common areas. Neither is 100% right or wrong, but making assumptions without discussing things can lead to problems. Last year you got lucky to have roommates more like you. This year you will have to consider the preferences of people different from you. It’s not a bad idea to learn to be more neat (and more courteous) in a group situation. Be casual in your own room (but don’t leave dirty dishes and food around that will smell and attract insects/rodents).