You can tell your friend anything. If you need to call someone late at night, you know there's one person who'll pick up. If you went to prison for justified though excessive assault, you and your friend wouldn't even bother to discuss his/her assuming legal guardianship of your child(ren). The level of trust between the two of you is rivaled by few.
Well, today's the day to fuck it all goodbye! After all, the only reason you two became friends is because when you met, he/she was either unavailable or found you too unattractive to screw and you've resented him/her ever since. But guess what, if you make a pass tonight, he/she will be in the no-win situation of either rejecting you and losing your friendship or banging you and losing your friendship. Since there's at least a one in a million chance that screwing will be your first step into a romantic relationship, as opposed to the absolute certainty of dissolution if you are rejected, the sex is practically guaranteed! Though both of you will be quite giving, almost to the point of competition, your coupling will be strained and mechanical. As if you're humping away with both of your brows furrowed with a look of "...the hell?"
But so what? It's sex, isn't it?