She didn't fall in love with you because you remembered her birthday and brought her flowers on Valentine's Day, She fell in love with you, because when you woke up in the morning, you said, 'Good morning,' to her before you checked your phone. She fell in love with you, because when you went to the fridge to get yourself a drink, you got her one without even asking. She fell in love with you, because when you had an amazing day at work and she came home and she had a terrible day at work, you didn't say, "Yeah, yeah, yeah. But let me tell you about my day." You sit and listened to her awful day and you didn't say a thing about your amazing day. This is why she fell in love with you.
I can't tell you exactly what day. It was no particular thing you did. It was the accumulation of all of those little things that she woke up one day as if he pressed a button, she goes, "I love him."