It's an Italian proverb. We generally say it referring to engaged couple (or married couple) who have the same temper. It has a negative connotation.
I see on tv that Pete Doherty and Kate Moss were arrested for drug possession and I say "Non si pigliano se non si assomigliano!"(or "Chi si piglia s'assomiglia") that means less or more "if they weren't alike they couldn't be together".