Choose and/or lose

Why is it you can see a relationship that will be a trainwreck miles ahead of those involved? Intuition? Foresight? Impartiality, or avoidance by the parties involved? A woman I work with is engaged. She had an accomplishment that was recognized by the organization I work for. They took her out for lunch and drinks, which lasted the whole afternoon. I show up a little after 5pm, so much drinking has ensued. The woman with the kudos is enjoying herself, but her fiance keeps tossing wads of paper at her. Three times she says "stop doing that, it's upsetting me", but this apparently is not a deterent. So now she is really pissed, and I'm wondering what this guys problem is. Turns out he doesn't treat her that great (surprise). I guess the point of my rant is why would people want to stay with people that bring them down? Maybe it's a self esteem issue, I don't know.