https://www.youtube.com/watch?v=C-VUZea36EI>You're friends seem nice.Check this out, man. Look how friendly they are at the end. Marceline and PB are chill, and even Jake is chill - "Later, Vampire Queen!".
It took seven seasons to reach this point. Seven. It took seven seasons for Jake to get over his fear of Marcy, and it took four (since it was first established that they had issues in 'What Was Missing') for Marceline and Bubblegum to finally bury the hatchet.
When we first met her, she was completely wild and emotional. A thousand years old.
Here's my point: She hasn't "changed", at least not physically, as far as we know. But she has -changed- as a person. She's changed through out the series. She changed at some point before the series. She will change in the future too. And so will all of the other characters, maybe in ways we never notice. We just never really reflected on that change. Very rarely is a show given that opportunity.
Think about your own life. Maybe you've gone years drifting, maybe in some dead end job or maybe as a NEET. Maybe you;re working on a degree. Maybe you lost your legs in a car accident one faithful night. Maybe you're about to meet the love of your life. Maybe you're going bald. Maybe not. Or maybe you're getting buff. Some things stay familiar for a very long time and you might think nothing ever changes, but those things will eventually disappear or break too. Nothing is exempt from cosmic entropy.
No matter what, it all still changes, ever so slightly. Daily and nightly. In little ways. When everything stays.
