By definition.
[Preliminary reading: Defining Love]
If God is love and is imagining future realities with us, a belief that God doesn’t exist is logically equivalent to a belief that love doesn’t exist. By imagining no future realities with God, they’re imagining no future realities with love. And if God’s love is perfect, this is perfect hate.
If God doesn’t exist in any of their potential future realities, then how could God be with them? Their future only exists where God doesn’t exist, because they do not believe he exists. And if he is literally this love that is the imagining of realities with us, how can someone that denies that exists experience it? Their denial of his existence doesn’t mean he doesn’t exist. Just like me denying walruses exist doesn’t mean that they aren’t real.
By our definition of love and hate, Atheists hate God. And God is love [I John 4:16]. So they also hate love.