Broken engagements

I know. Divorce, Broken Engagement. Doesn't sound very optimistic. But let's face it. Life doesn't really give us reasons tobe optimistic...

I have this friend, she dated a guy for 3 months. She told me how different it felt from other guys she dated, and how so many things could have been fatal to their relationship and somehow, everything worked out so perfectly. How previous dates should have worked and didn't.
To cut a long story short, he proposed. She agreed. They got engaged. She got this glowing look and happy smile all brides-to-be have. She talks about the impression of feeling whole, and how everything was MinHaShamayim etc...

And then... A week or so after they got engaged, falls the terrible news.
They broke up.

I have this other friend ( not a close one, but still a friend) who dated a guy in the summer. Things weren't perfect, and after a while, they called off the Shidduch. So far, so good. But then, comes December, and they give it a new try. They are going out again.

Bingo ! They're engaged !!! They were set the big day for tonight (Tuesday).

And yes. On Thursday I heard they broke up. And on Sunday I heard they're back together and are getting married on Tuesday. And Monday I heard it's really over. And now it's Tuesday, and there's no wedding.

How can it be that two people date, enjoy each other's company, like each other for a long while, and when they finally make the big move, everything falls apart?

What is it in the engagement process that breaks the bond, the connection that was created during the dating period? Is it scary? Is it weird? Is marriage scary? Horrible?

Please, you people who went through this, tell me why this happens? Or that it doesn't really happen, and some people are actually happy....