Sebastian Schipper - Ein Freund Von Mir AKA A friend of Mine (2006)

Karl and Hans couldn’t be more different. Karl is a young mathematician with a promising career at an insurance company, whereas the man-about-town Hans only takes up the odd job to get by. When they meet, Hans asks Karl whether he is happy. Karl doesn’t know what to reply until Hans shows him what makes him happy: girls, airplanes, the fastest backwards driving car in the world, and driving a Porsche at night in the nude along the Autobahn for example.
For Hans “friendship” means to share everything, even Stelle, the queen of his heart. This is too much for Karl. But then, you can’t just get rid of a friend like Hans, and a woman like Stelle is unforgettable...

It might sound a bit childish and naive, but it's one of the most lovable films I've seen in quite some time.