Everything explained (SPOILERS)
Q. Why does Franck make a beeline for Henri sitting on the shoreline, despite giving no indication at any time that he is a chubby chaser?
A. Because he is FRENCH. Liberté, Égalité, Fraternité!
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Q. Why does Henri spend his entire three-week vacation sitting by the lake, staring out into space, then decides to dangle himself in front of Michel like so much psycho bait and resigns himself to being killed?
A. Because he is FRENCH. C'est la vie!
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Q. Why does Michel go around killing people?
A. Because he is FRENCH. Comme ci, comme ça.
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Q. Why does Franck not go straight to the police after witnessing the drowning and comes right back to the lake to carry on swimming and cruising after Michel?
A. Because he is FRENCH. Oooh la la!
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Q. Why does Franck initially hide from crazed killer Michel, then start calling out to him?
A. Because he is FRENCH. C'est si bon!
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You'll find this works for pretty much any arthouse French movie.