How do you know you've found the best possible person for you?

Reading all the comments on @daloon’s question led me to think about this.

I know people that have “found” their SO, sometimes believe him/her to be “the one”. My question to these people is – what led you to believe that they were it for you? I’m not asking how you know you loved them, I’m asking how you knew there wasn’t someone else better for you.

Though I personally don’t believe in “the one”, I do believe that different people are compatible for us in different aspects of our lives. Ideally, you try to find someone that is compatible for you in all aspects of your life, someone with whom you share the same ideals, values, hopes, goals, and dreams.

So maybe you’re 30, and you’ve finally come across someone that tops the charts, the most compatible person you’ve met in your lifetime. Is it not possible that were you to stay single for another 10 more years, you’d meet someone else even more compatible for you?Just because they’re hard to find, because you haven’t found that person yet, doesn’t mean they’re not out there.

I’m not disputing the fact that you could be happy for the rest of your life with this person. I’m just curious about how you know that they’re IT for you, and that’s the point where you stop looking. How do you know someone else out there couldn’t make you even happier?

When you’ve lived life seeing people from 1–10, how are you certain that the scale ends at 10, and that you won’t come across an 11 some day?