1. There is no such thing as "one true love". 
There are so many people in the world, so many personalities, in even the teensy portion of those that any one person interacts with there are several matches that can be made.  That's why people in a group will sort of trade partners every so often.  Yes, it does happen, I've seen it.  Every person has different sides to their own personality that mix pleasantly with others of different types.  The odds of finding one person who can match, oppose, compliment, balance, and please you in every way are so small that it is unreasonable to think someone claiming to have found their "one true love" couldn't have been just as happy with someone else if they had met that person first.

2. Most relationships fail. 
Statistics, right?  How many relationships have you been in versus how many are you still in.  Find a couple that has been married over ten years and ask them how many people each one dated before they were wed.  Odds are most people will have "tried on" more than one person who didn't "fit" and eventually found one that did.  The rare few who married their first high school sweetheart?  Well, they are either lucky or lazy.