Personal boundaries are the one thing I struggled with most throughout college. Not that college is a specific time and place for you to realize and have personal boundaries, but for the last few years of your teens and the rest of your twenties, you should start to understand why personal boundaries are important.
If you are unaware of what personal boundaries are they are the things that keep respect between all of your close and/or personal relationships. Boundaries are your list of do's and don't's that should be given out like a pamphlet when you start to build a relationship with someone. They are what keeps people from walking all over you and they are what keeps you feeling like you and your values are respected. They can look like developing trust over time instead of the moment you meet someone. They can sound like, "no" or "I'd rather not". As they feel comfortable for you, they can feel very uncomfortable for others. And this is important to keep in mind when standing strong to your boundaries; The uncomfortable feeling that others feel regarding boundaries that keep you satisfied in life is a personal problem of their own and has nothing to do with you or what you want your life to look like.
One thing that I, and I know many others, have struggled with is staying true to our own boundaries. There are certain things we do not and will not tolerate in our life, even things done by those who we love and adore. Allowing others to cross over your boundaries with no consequences shows them that you, too, have no respect for your boundaries. When you are wondering why someone would say something or do something to you, understand that you have let them cross a line that you should have been stricter about in the first place. And that is a problem that we all have - not voicing our boundaries from the beginning.
Not until the time comes do we say, "I don't like when you do that." When something has happened over and over again and you have been pushed to the point that you feel comfortable to say how you feel. This is what causes you problems and these are problems that you can only blame yourself for. What you allow is what will continue. And over time, you will notice that you have lost track of any and all boundaries and that you are unable to allow healthy, personal boundaries within any close relationship you have. But if you voice your boundaries and you stick by them the way you want others to, then you will keep your sanity in the end. Your relationships will value respect and individualism. You will learn that you are allowed to choose the life that you want to live and you are allowed to choose what is wrong and right in your life, and those who want to be in your life will do so by the rules you have set for them.
Also, when you want your boundaries to be respected you have to respect other's boundaries. But the cool thing with that is that people, like yourself, can walk away from someone if their boundaries conflict with yours.