Dr. Lolipop
You know you're in love with someone when you can see them surrounded by other people and only care about smiling at them, or sharing an inside joke. To put it simply, that person is all that matters.
The usage of "fall" shows helplessness, which is true, as you can't help falling in love. Also, people tend to describe love differently. For instance, a person sees a beautiful girl, and says, "Damn, I'm in love", but other people won't think of it as love but mere infatuation.
The definition of love may vary person to person but it's more or less the same thing, in my opinion.
Antony
Honestly it's all just about what you're feeling and how you interpret those feelings. It varies from person to person.
Kevin
it means you fall in love with a person
ra†ia
Here is the link...
Pink Martini
[x]. http://youtu.be/Ldxn6aq2GCc
Edit: http://youtu.be/9Jz706sJMjg
http://youtu.be/CVJwbclo-II
http://youtu.be/z7qop0CGHAo
http://youtu.be/6nZGv8VTBVE
http://youtu.be/zu8eStPNTvw
http://youtu.be/X-idP23bHCg