You've just started to form attachments to people, and suddenly, it's time to say good-bye. They've become familiar to you. They're a part of your life. Ve ry soon, there will be nothing but a void. It won't be left empty for long, however.
New friends fill the space left by the departing seniors.
Maybe, with the passing of time, you will lose some of the memories. But you can't forget them. They were a part of your life, if only for two years. Think about whom you're going to miss from the senior class. The more people you think of, the harder it is to let them go.