Why do lovers become strangers?

It's a pretty hard thing to comprehend for me anyways. You meet a girl. Spend some time together. Go out. Enjoy the time that you're with them together. It might be a month, 6 months, 2 years, and so on. Most of the times it ends good. You live the fairy tell that we've always heard about. However, most of the times you end up breaking up.

You break up. Things are said. Sometimes it's good and sometimes it's ugly. You accept that fact and there is nothing else you can do about it. Now for the part I don't get. You two shared maybe some of the best moments in your life so far. You enjoyed each other's company. You were best friends.

Now after the breakup one of the partners acts like it never even happened. As if it never phased them. On to the next guy as if she's changing her shirt. Hell, you sometimes even see the person at a mutual friends house. You say your hellos but then go your separate ways. Like two strangers who had never spent anytime together before.

How can someone not be affected. I'm pretty sure 2 years down the road if I saw her walking down the sidewalk, she wouldn't even acknowledge me. It's a hard thing to comprehend. At least for me anyways.
Why do lovers become strangers?
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