Goodbyes are the worst.
No matter how long you've known the person, and how strong the relationship is, goodbyes are always hard.
Goodbyes are especially hard when you're a young adult. Early adulthood is when there are a ton of changes. You graduate high school, go to college (locally or far away), start a career, make new friends, date- you know how it goes. Since everyone is going through so many changes at the same time it can be difficult to know if those relationships will continue, or if they'll fade away as time goes on.
Tears flow as the grief squeezes your heart, and everything that is in you, sucking the air out of your lungs and leaving plenty of room for the ache that remains.
And you unfortunately cannot feign the 'see ya later, this isn't goodbye', because you know that this probably is your final goodbye to some of the people that you have had in your life.
But don't give up on relationships too quickly. There is still good in them, even when some end in painful goodbyes.
The more painful the goodbye, the more you probably got out of the relationship.
Even though goodbyes are the worst, they also mark new hellos. So be open for those hellos, and be ready to greet them with a smile.