Why does love hurt?

I'm so sad. My brother and sister and many of my friends have ended up falling madly in love and engaged or married to the first person they started dating. I've fallen in love and been heart broken 3 times now and I just don't understand why it keeps happening and why I couldn't have ended up living happily ever after just after one attempt like they have. Is there "the one" out there for me?? Will he be obviously different and will the love be undeniably stronger to where I KNOW he's the one and we'll end up together?? I don't want to fall in love again because I'm so terrified I'll just end up heartbroken, devastated, and torn up from it once again. 
Whoever has been in love multiple times and is now married-- is your current SO obviously the best one in your eyes? Is there something different with him than your previous loves? Or, could you have probably married the other guys beforehand if circumstances were just different? Please share you experiences.