This is only my point of view of what happened, so I'm sorry if I come off as,,, idk weird
Okay but what happened to Steven was so damn important and it hit home so much it hurts because I feel the same HAH.
Look, I know many of you probably are thinking that there was no reason for him to react that way,,, but there was!
About Sadie and Shep,,well, that's Sadie's private life, so I understand that maybe she would only tell Lars about Shep and the band since they are her closest friends.. Ita kind of important to her, but not THAT much for Steven.
But then knowing the band split up? And not knowing about it before? They never told him about it and left him in the dark about it until it split. They are Stevens friends too, and a decision that important should have been told. At the start of the episode you can see him enjoying Sadie and the killers music, completely unaware that they were separated, and I'm sure he would have liked to know about it sooner. None of his friends bothered to tell him anything.
The same with the off colors. For the love of god, Steven considered Lars a friend since season 1 and his bond with him grew stronger and bigger as the episodes went by, and Lars only tells him he's going to space again like,,, a day? Hours? Before going off? I don't know about you, but I'd be really pissed if a friend I love didn't tell me about it sooner. Too little time to talk and hug and be with your friend at least one last time.
And then that,,,, thing. That thing you feel when you know you're being left behind because everyone is moving on except you. And I feel it. I feel it so fresh and bloody and so close to home it hurts, because I'm going through the same.
He feels left behind because no one is talking to him, communicating,, actually trying to comprehend what's happening. (His fault too, for bottling everything up, but now we know why he does it)
When was the last time we saw him talking with Sadie? Or the off colors? The cool kids? Lars?
Steven needs someone to talk to him like the old days, but also remind him that this is the future he doesn't need to be needed, he needs to find his purpose in life and at this point, isn't that what teenagers and young adults struggle with the most? Finding what you want for your life? For your future?
I really think this new series is going to be about him trying to heal about the past and start looking to the future.
Tl;Dr : steven is being left behind bc he can't move on from the old beach city. Old friends, old problems.
(This has some loose things so please add on your thoughts, it's fucking 05:27 AM)