When we meet the right person to love, when we're at the right place at the right time, that's chance. When you meet someone you're attracted to, that's not a choice. That's chance. Being caught up in a moment (and there are a lot of couples who get together because of this) is not a choice. That's also a chance.
The difference is what happens afterwards. When will you take that infatuation, that crush, that mind-blowing attraction to the next level? That's when all sanity goes back, you sit down and contemplate whether you want to make this into a concrete relationship or just a fling.
If you decide to love a person, even with his faults, that's not a chance. That's choice. When you choose to be with a person, no matter what, that's choice. Even if you know there are many people out there who are more attractive, smarter, and richer than your mate, and yet, you decide to love your mate just the same, that's choice.
Infatuation, crushes, and attraction come to us by chance. But true love that lasts is truly a choice. A choice that we make. Regarding soulmates, there's a beautiful movie quote that I believe is so true about this: "Fate brings you together, but it's still up to you to make it happen." I believe that soulmates do exist, that there is truly someone made for you. But it's still up to you to make the choice if you're going to do something about it or not.
We may meet our soulmates by chance, but loving and staying with our soulmate is still a choice we have to make. We came to the world not by finding someone perfect to love, BUT to learn how to love an imperfect person perfectly.