One thing I've learned as an adult that I think is challenging for younger people to understand is that not all relationships are meant to last forever. People will come and go from your life and this isn't always a bad thing. People grow, people change, and everyone is after their own agenda. Friendships don't always have to end on bad terms either. Sometimes, people just talk less and less as time goes by and it eventually stops all together. As you get older, friendships become harder to build. Everyone is so busy all the time. This doesn't mean you shouldn't value the relationships while they are active though. Putting the effort in is really important. It's important to let other people know they mean something to you.
Have a good day friends! <3