This is when...(in my opinion...)

1) You think of the person endlessly.

2) Just seeing them, even if it's from a distance, makes you happy.

3) You feel weak at the knees when you see them.

4) You just want to be with them.

5) You'll do things you wouldn't just to speak with them.

6) You'll make any excuse to make them smile.

7) They make you feel better, you make them feel better.

8) You trust eachother.

9) You can't imagine your life without them.

10) Everyone else in comparison is nothing to him. (Or her xD)