It sucks because you feel like something has been taken from you. I was pointing out that is unfair to claim rights on other people's relationships, usually it just means that we are still egoist (immature) about relationships, that we see still as a way to have love from other people. And I'm talking also to the 5 who downvoted me. :)

Just think about this: when we are children we don't care about who we love: we don't care about waking up our mother in the middle of the night crying for a hug, and we NEED to be loved. Usually this lasts for 20 years, in different form. After that, we learn to be emotionally self-sufficient, and become more badass. In this phase, we learn to provide our love by ourselves. We have many friends/partners, enjoy them, but don't NEED them anymore. We are happier if they are happy, and it's not a problem at all if two of them form a bond (that inevitably cuts ourselves out a little bit), but we enjoy their joy. After that, we're so selfconfident that we can start giving love to other people without having anything back, just because giving love is our basic mindset. In that phase, we attract people and feed them love.

Don't be angry when I tell you to grow up. Just think that you should be happy if they are happy. Even if they are happy without you. And if you are not happy, probably that's just a little bit of egoism. Nothing serious, but something you can work on! ;)