Party god definitely has an interesting back story that no one talks about. Anyone notice the connection between him and the bears in Belly of the Beast?
Marceline's first words when Finn was in the lute suit outside her home weren't an immediate "No!" Instead she appeared shocked and told Finn "Trust me, you dont wanna date a girl like me." and proceeded to transform into a monster. It seemed more like she felt undeserving of love, even by someone with whom she feels close.
Just because a girl says she wants to be friends doesn't mean she isn't interested.