According to Huffpost

1. It’s something inside.

2. Flashbacks.

3. You just get each other.

4. You fall in love with his (or her) flaws.

5. It’s intense.

6. You two against the world.

7. You’re mentally inseparable.

8. You feel secure and protected.

9. You can’t imagine your life without him (or her).

10. You look each other in the eye.

See more at Murphy, Sam & Jody.