Not everything is supposed to become something beautiful and long lasting. Sometimes people come into your life to show you what's right and what's wrong, help you discover new things about yourself, make you feel better for a little while, or become a lesson you won't ever forget for as long as you live. Not everyone I gonna stay forever, so thank them for sharing their life with you and move on. Similarly, be nice. You won't always be with that same person, nor do you know how they'll impact you.