"It's not goodbye, it's see you later."
Personally, this is my favorite phrase. You never know when your last day is or anyone else around you for that matter. "Goodbye" implies that you will never see them again, but "see you later" means you will.
"See you later" says this is not the end. It says there will be a tomorrow. It says "I have to leave now, but there will be a next time." Even though tomorrow is not promised, it does not hurt to have hope that there will be a tomorrow. Hope is what keeps me and many other people going. Goodbyes are like saying there will never be a tomorrow or a next time and this is the end. Goodbye is only for people you are never going to see again.
When you are used to people saying goodbye and leaving you, it can be difficult to have hope that not everyone is going to do the same. Saying "see you later" is sort of a consolation, a promise that they are not going to leave–that this is not forever. Life is fragile and you never know what is going to happen, whether it be a disease, car crash, or something far worse.
A goodbye is short. "See you later" is one verse ending until the next time where a new verse begins. Whether that be five minutes later, a couple months or even years. If it is a loved one that has passed away, even though they may not physically be with us, they are not gone.
In the words of Peter Pan,
"Never say goodbye, because saying goodbye means going away. And going away means forgetting."
No one is ever truly gone until they are forgotten.
Whether you are a college student going away for school, or graduating from high school or higher education, or someone who lost a person close to you, always say "see you later." Anything can happen at any second of any day, so it is best to say see you later–in whatever way that may be–than to say goodbye and regret it later on. It is never goodbye.