Because more often than not, high school relationships aren't made to last. Teenagers are hormonal and can often make the worst choices. It's why Romeo and Juliet isn't the greatest romance, it's a story of two dumb teenagers making a series of bad choices.
College IS a time of stress and freedom, but it's also a time where people actually mature and figure out what things they really want, and if they want to choose a relationship or higher education. However if people are mature enough and their feelings are strong enough, they can talk things out and make it through college. Because let's face it, if you go to a party and you hook up with someone because they're there and your partner isn't, methinks your feelings for your partner weren't so strongly based on love but more a case of lust.