I feel like something that’s so essential to dean and cas specifically deans feelings for cas isn’t necessarily how well cas knows dean (obviously that’s a big and beautiful part of it) but cas’s choice to be on deans team. So much of the beginning of the show comes from dean chasing his family down. Being the one you can rely on but never the one who relies on you. Chasing down Sam for help with John, chasing down John to help him, chasing down the demon for his dad for his mom, being the one others call, watching people fall off the map while he stays put with his job. And very alone in all of it. And cas saying we’re making it up as we go is something dean can relate to to such a degree it actually rocks me to watch that scene. Someone like castiel (someone at all) standing beside him and choosing to travel down this lonely road by his side for better or worse. Someone he can rely on and relate to, a true partner. Not tied down to him by blood or oath but by choice. Who cas is just goes with who dean is. They just. Go together. They want the same things. Their worst conflicts happen when their shared road diverges and they can’t follow each other. WE are making it up as we go. Me and him. They go together. Best friends. By choice.