I have no explanation as to why Life is this way. I'm sure religious people would tell you that Life is meant to be a test for us (Losties will probably agree), but I wonder if it's not instead the reverse: that Life came first, then came Society, which set up the complicated rules and invented the good-but-bad-for-ya stuff---like Krispy Kreme doughnuts, Frosted Flakes, and crack.
Rather than look for the answers to life's mysteries, I think you'd be better off for now just accepting the fact and being aware that it just can take just seconds to drastically change your life. A few examples:
- Choosing to drive while drunk and getting into an accident
- Choosing to try anything addictive "just once"
- Choosing to have unprotected sex
- Choosing to cheat in any way
All you have to do is be aware that a second can change a lifetime with no second chance and I think you'll be more apt to take your time to make better choices and have a better life. Simple!