What is love?

What is love? Does it measure how long u're together?or how well u know each other?

For the 1st time in my life i saw the loss he felt or he will feel if i'm gone. And i'm not expecting that. He's such a 'cold' guy.I never think he will love me that much. Or he will feel that much lost if i'm gone..

It kinda shocked me..and it kept me thinking...

What is love?