There is a military adage used when calculating an opponent's intentions: "capability implies intent." As a sysadmin, mechanic, carpenter, or skydiver, if you're at all sane, you'll plan for equipment failure.
I keep a pair of MRE's in my trunk, with road flares, stout rope, jumper cables, and some other stuff that might not belong there, if I should ever get in an accident. So, yes, I technically have a backup. But the word "always", well... all generalizations are bad.
Have a backup when you're charging an enemy foxhole. Have a backup when you pick a restaurant on Friday and you haven't made reservations. Have a backup when you're planning an elaborate prank. But love? No way.
Love is about trust. The risk of terrible heartache is what makes love sweet. It's what makes a passionate kiss more than a peck, what makes spontaneous road trips so fun. It's about living in the moment, enjoying what you have, and not thinking about what you will have or what you might not have.
I had a backup. We said that if we turned 30 and hadn't married anyone else, we'd go ahead and get hitched. It turns out that she didn't know the first thing about trust; I caught her reading my journal, and ever since, well... that's not the kind of friend I keep. The girl I ended up with instead--the one I almost didn't get together with--has been my best friend, my lover, and all the backup I need, for the last three years.1
1. Node for the ages caveat : we're married now.