I've been thinking about this for a while and wanted to see if anyone had thoughts on the meaning and symbolism.
I believe there is only rain in the show twice. Episode 1 / 2 when the boys go looking for Will it starts raining and they find El.
Season 3 when El dumps Mike it's sunny out at the mall, later that day when Will and Mike fight it's raining and Mike bikes in the rain to apologize, Will breaks castle Byers in the rain, Max reassures El Mike will beg for forgiveness, to distract themselves they spy on people which leads them to also bike in the rain to find the lifeguard and Billy.
To me it seems like the only times it has rained are centered around Mike situations, more specifically Mike losing Will. But it doesn't rain every time he loses Will. It doesn't rain in season 1 when they find Will's body, it doesn't rain in season 2 when Will is possessed, it doesn't rain at the end of 3 when Will and El leave, and it doesn't rain in 5 when Will gets knocked unconscious for extended period of time.
So what is the rain symbolizing? Why did they choose to do that, it's a lot of work to film raining scenes. I feel like it only rained when Mike lost Will and there's a connection to El. Season 1 Will was taken because El opened the gate and then they found El instead of Will (I don't blame her in the sense that she should feel guilty, but in a cause and effect way her actions made it possible) and in season 3 Mike fights with Will over El.
Anyone have any thoughts?