Falling in love tales time but once you fall in love, you are in love and then you feel the person you felt in love with is one of a kind and so on... Falling in love is quite dangerous, but sometimes it can make you really happy. Well, provided it isn't unrequited love. If it is unrequited, you suffer a lot. Then you're not on cloud nine ;-)
Definition: to begin to love someone.
Example: I think I'm falling in love with him.