Personally, I think it depends on many factors. How you love, what you believe, how things work out. My girlfriend and I work so well, I think, because we take love as we take a really intense friendship. There isn't as much of a distinction as I see some of my friends have, and that's important to me, and why I think Zo and I are more likely to get through stuff than some of my friends are. I can go to her with anything- there's absolutely nothing I wouldn't talk to her about. And then I see my friends talking about how they can't tell their boyfriends about this or that or something.
For some people, the way that she and I are in love wouldn't work. Some people have to have that great love romantic thing... I've dated people like that, and even though I've loved them, it wasn't really the kind of love that I could stick with. I'm young, and people don't seem to believe anything romantic can last because of that, but I know what sort of love fits me, and it's the friendship sort.
How this relates is that I think there are two kinds of first loves. There is the first time you ever love someone, and I think that's rare that it'll work, but very, very possible. And then, there's the first love that makes you see this whole other level to things and makes things click into place and makes you look back on your old "first love" and laugh because it wasn't as life changing as you thought. And maybe some people can have both loves at the same time. And maybe some people never get that second kind of love. And sometimes, if you get that second sort, people won't take it seriously.
I will always love my first (in the first definition) love, but not in the same way, and I think it's more that I love those memories- the person honestly annoys me more than anything else! But I couldn't marry her. Never.
I don't want to go into chances and percentages. Everyone is different. My aunt and uncle and my grandparents were both first love marriages who have stayed together. I think people are too cynical about how there's this little to no chance that you'll always be with the first person you love. It's not a huge chance, but it's very there. And a lot of the time, things can be prevented. I think that a lot of people don't want to put the work into working to stay together and just let things fall apart.
Well, that's my view.