Daisy is leaving you for me, so you won't be able to "take care" of her anymore
Daisy would never leave me for someone like you
Gatsby vs Nature
Why does it have to rain on the day I've dreamed about for years
Daisy is Gatsby's American Dream, so he does not want to have to let go of her. Tom is getting in the way of Gatsby's happiness with Daisy. Gatsby's only dream would affect many people's lives, but he is too blinded by his hope to see that.
Gatsby had been waiting for the day that he would see Daisy again for years, but on the day the weather was bad. Gatsby is drenched by the rain before getting to see his love.