That question was asked of me today. That's a tough one. I knew when I fell in lust with him (I know that will completely gross out anyone under the age of 30 who would happen to read this). That was when I was still living with my old boyfriend and his family. Ironically my boyfriend's father got me the job where I worked for John, I then became friends with him, fell in love and married him. And I have to say the last 26 years have been pretty good.
When do you really fall in love with someone? in my humble opinion--
When they are one of your best friends (but ladies, you will ALWAYS need your girlfriends. Always.)
When you miss them, but are confident enough to know that they can have a good time without you - and when the feeling is mutual. Time apart can be good.
When you just know when to give in, and when to stand your ground.
When you agree on the not-so-little things... food, music, TV shows, movies...
When you want to tend to their needs when they are sick (And guys - it means A LOT when you hold a girl's hair when she gets sick. Hopefully this will only occur when she has the flu. I'm just saying.)
When you start completing each other's sentences.
When being around them feels more natural than not.
That's it. That's all I got. Maybe it will help,
but probably not.