Stranger Things rant incoming:
The criticism of Will's behavior towards Mike during the rain fight in S3 is so infuriating for so many reasons (many of which have already been articulated repeatedly on this platform for multiple years), but something I always think of when people say "Will didn't care about Mike's problems" or "Will bottled up his anger and never made any productive steps to resolve it," is clearly none of you have played Dungeons and Dragons before, because if you had, you'd know yes he fucking did.
Mike is the designated Dungeon Master for their party, which is a lot of work/responsibility. As a DM, it's essentially your job to not only write a compelling story with interesting adventures and battles for your characters, but to create possibilities. You create various non-player characters, some of which your players may never even speak to, get familiar with tons of monsters' stats and behaviors that the players may never fight, develop traps and interesting objects to challenge particular characters and engage your players (who probably all have different styles and aspects of the game they enjoy), and often improvise these elements entirely on the fly when the players inevitably do something unexpected. And you have to do all of that, while doing the appropriate math to keep the goals for the characters reachable and challenging. Everyone's enjoyment of the game is resting on your ability to write, prepare, think creatively on the fly, and engage with players collaboratively and not combatively. That's why, when the DM has to delay the game or takes a little longer to prep a session than expected, people are forgiving. The whole game rests on their ability to do ten times the work of everyone else involved.
So when Mike gets distracted by his girlfriend and has no time for the party anymore, and his friends understandably feel rejected, Will's decision to write his own campaign and DM is actually, contrary to popular belief, an incredibly proactive and understanding step towards mending some of the relationship fractures he's seeing. Rather than walk up to Mike and say "Wow, with all these changes in your life, you don't have multiple days of spare time to spend writing stories and scenarios for us?" Will makes a far more empathetic move imo, saying "Well, if Mike doesn't have the time to write a campaign, I can take up the mantle for a bit, and then we can all spend an afternoon playing together." His demand on Mike goes from "spend days, possibly weeks, of your time curating a day's experience for us" to "I'll spend days, possibly weeks, of my time so that I can have one day of yours."
So when Will ambushes Mike and Lucas and essentially says "hey, play this game I've made for you," it's not the same as bringing over an old board game and demanding your friends humor you (though, as others have pointed out, he wouldn't be at all unjustified in that request given how little time they've spent together + everything Will's been through). He's put painstaking effort into not only quality time, but relationship repair for the three of them, only to be met with outright mockery from Mike and Lucas.
(It's also worth noting that the amount of time and effort this requires is a spectrum, and creating the "really cool campaign" Lucas and Mike both insist it was likely would've placed Will's at the upper end of it)
So if, somehow, you were still wondering whether Will's bitter feelings over this are reasonable, think through the empathy and maturity it would take to set aside months of neglect, spend hours of your time on an easily accepted compromise, then be rejected and treated like a child by two of your best friends in the world, and not immediately lash out.
Will Byers is a better fourteen year-old than I ever was.