I think it helps that though they liked and even love each other they were never really in love. Big difference.
Nanase dated Elliot as he was as close to the ideal man as she could find among her friends and acquaintances. Elliot on the other hand dated her because he liked her as a friend and when she approached him he really didn't understand the feelings of love and being in love. When they broke up neither of them felt that there was a big gaping hole where the other used to be, and so there were no hard feelings.
Sarah and Elliot dating was similar. They've known each other since they were toddlers, and though there were friendship and love, and some lust due to hormones, they were never truly in love. The breakup again showed that neither of them suffered the trauma of having their heart broken, nor were there much anger involved. There were some frustration but not much anger.
In both cases some feelings were a bit bruised, but no hearts were broken... The fact that none of them is a selfish a-hole does help.