The only way this happens is if one truly believes there is only ONE soul-mate. If their mate dies they already feel dead themselves. It's not difficult to imagine such a person being open to killing themself. (Giving up on life)
However in reality with close to 7 Billion people on the planet the odds are there are thousands if not millions of people who would "get you" or make a perfect partner. While the "one soul-mate theory" is a romantic one it's also usually the thought of (an immature person or someone lacking much relationship or life experience.) This explains why it's usually people who are young or a person who has only been with one or a few people for a long period of time that buy into the Romeo & Juliet type of love. (Eventually every new person you get involved with looks like they are "the one" until you realize they're not.)
After you have broken up with your high school sweetheart, your college sweetheart, dated a few others as an adult you begin to see "the one" at age 18 may not be so ideal for you at age 25, 35, or 45. Each of us evolves in our thinking and desires over time. Believing there is only ONE person on this whole planet for you without ever meeting (a fraction of the 7 Billion people) out their is almost like believing the world is flat.
Your perception becomes your reality.
Some people would rather give up then keep trying....