In college, roommates have a lot to deal with: in addition to the stress of being in school, you're squished into a space that would be incredibly tiny for one person ... not to mention two (or three or four). Just because you're sharing a space, however, doesn't necessarily mean that you're sharing all of your stuff, too.
As the lines begin to blur between where one person's space ends and the other's begins, it's not uncommon for roommates to begin sharing things. Why have two microwaves, for example, when you really need just one? While some things make sense to share, however, others can create a conflict.
If your roommate has started using your stuff in a way that you don't like, hasn't been talked about, or was previously talked about but is now being disrespected, a simple act can quickly turn into something much bigger. If your roommate is borrowing (or just plain taking!) your stuff without checking with you first, there are fortunately some questions you can ask yourself when trying to figure out what to do about the situation:
How big of an issue is this for you? Maybe you talked about sharing items and your roommate has disregarded the agreement you made together. How much does that bother, annoy, or anger you? Or does it make sense that he or she used your things without asking? Is it a big deal or not? Try not to think about how you think you should feel; think about how you do feel. True, some people may not care if a roommate borrows their iron, but if it bothers you, then be honest with yourself about that. Conversely, if your friends seem outraged that your roommate borrowed your clothes but you don't really mind, then know that's okay, too.
Is this a pattern or an exception? Your roommate might be absolutely great and she took a little of your cereal and milk just once because she was super, super hungry late one night. Or she may take your cereal and milk two times a week and now you're just sick of it. Consider whether this is a small incident that likely won't happen again or a larger pattern that you would like stopped. It's okay to be bothered by either one, and it's especially important to address any bigger issues (e.g., the pattern) if and when you confront your roommate about their behavior.
Is it a personal item or something general? Your roommate may not know that, for example, the jacket he borrowed was your grandfather's. Consequently, he may not understand why you're so upset that he borrowed it one night when it was unseasonably cold. While all of the things you brought to college matter to you, your roommate doesn't know the values you assign to everything. So be clear on what was borrowed and why it's not okay (or totally fine) for your roommate to borrow it again.
What bugs you about the situation? You may be bothered that your roommate took something that you told him not to; you may be bothered that he did it without asking; you may be bothered that he didn't replace it; you may be bothered that he takes a lot of your stuff without checking with you first. If you can figure out what bugs you the most about your roommate's use of your stuff, you can better address the real issue at hand. So sure, your roommate may have a reason for taking your last energy drink, but it's harder to explain why he is constantly helping himself to the last of your things.
What resolution do you want? You might just want an apology or an acknowledgement that your roommate took something he or she wasn't entitled to take. Or you may want something larger, like a conversation or even a formal roommate contract about what it's okay and not okay to share. Think about what you need to feel better about the situation. That way, when you do talk to your roommate (or RA), you can be focused on a larger goal instead of just feeling frustrated and like you don't have any options.
How can you best come to a resolution? Once you figure out what kind of resolution you want, it's important to also figure out how you can get there. If you want an apology, you'll need to talk to your roommate; if you want clearer rules in place, you'll need to think about what those rules might be before starting a conversation. If you can take the time and mental energy to focus on the causes of and solutions to the problem, your roommate's use of your stuff doesn't have to be anything more than a minor issue you thought about, addressed, and resolved during your time as roommates. After all, you both have much bigger things to worry about ... and enjoy!