when you fall in love with someone, say your best friend, and it doesn't work out because you're her "Friend". to her, you're just friends.
A woman has a close male friend. This means that he is probably interested in her, which is why he hangs around so much. She sees him strictly as a friend. This always starts out with, you're a great guy, but I don't like you in that way. This is roughly the equivalent for the guy of going to a job interview and the company saying, You have a great resume, you have all the qualifications we are looking for, but we're not going to hire you. We will, however, use your resume as the basis for comparison for all other applicants. But, we're going to hire somebody who is far less qualified and is probably an alcoholic. And if he doesn't work out, we'll hire somebody else, but still not you. In fact, we will never hire you. But we will call you from time to time to complain about the person that we hired.
"just friends" example from bash.org
when you discover you're in love with your best friend.
possible scenarios of revealing your best friend love.
reply A: oh <insert name here>, i've had feelings for you too... let's be lovers. which will go on to the best relationship you've ever had.
reply B: what?! but you're my best friend! how could you have feelings for me in that way?! i feel so betrayed!! which will lead to a chilling of your relationship, leaving you wishing you just kept your mouth shut. it will normally get better alter some time passes, but it'll never be the same.