Changes in communication, lots of conflict, a decrease in the amount of time you spend together, and, of course, physical or emotional abuse are all reasons to end a relationship. You might also find that your - or your partners' - goals or interests, emotional involvement, or expectations change.
Sometimes, even though a relationship is clearly unhealthy, partners would rather stay together and be miserable than break up and be "alone". This is a huge pitfall to consider when you're dating - staying with your partner simply to avoid being alone can be extremely unhealthy and rarely helps you to grow or gives you the true satisfaction that you deserve in a relationship.
One of the toughest things to go through is a break up - especially if you’re the one getting dumped. Suddenly, homeroom - the place you've been every morning for how many months? - is awkward and strange, the McDonald's around the corner is painful to see, and forget those two classes you share. Before they were the highlight of your day...now it's almost impossible to sit through five minutes. Unfortunately, this "getting re-used to things" is a part of the break up that, while painful, is very necessary. Even though meeting new people may seem about as fun as a root canal, new relationships will develop in time.
Breaking up with someone is never easy...but it's also probably not the hardest thing you'll go through. Lean on your family, your friends, and your hobbies during the tougher times - you'd be surprised how much it'll help to talk to people who love and care for you or to distract yourself with books, movies, working out, playing sports, etc. It doesn't seem like it now, but remember - this feeling won't last forever.