The Aliens aren't evil. They just wanna party. Then why shouldn't we let them party in our party place? BECAUSE WE WANNA PARTY. And WE aren't going to let those aliens take our party place.
After living the life of no partying the aliens travel to Earth to end their misery. They target the best party there is and take over.
Note: Even though we are against them they are loved by Party for their will to PARTAY.