I hate it when people beautify
dying for the person you love. To treat life as something so fickle disgusts me.
a) They left you willingly. You lived without their love before, you can do it again. If it's meant to be, it's going to be. If it isn't, you're going to heal, become stronger and move onto brighter days. There is nothing romantic about ending your life because your relationship failed.
b) They left you unwillingly. The most obvious case I can think of is if someone you loved died. If that's the case, think about what they wanted for you. If the roles were switched, would you want them to commit suicide because they couldn't be with you anymore? NO. You would want them to keep moving on in life. You would want them to learn more, experience more, and love again. You want the best for them.
If someone left you, look for someone who won't. If you've lost someone, let them live through you.

"We don't die for our friends. We live for them."
                                                                                    ~  E r z a   S c a r l e t