What ridiculous answers you have got-unsual really
Maybe it is that this is the only person in your life that you can really trust and get on with-so when you think about him with other people-him giving them attention instead of you-and sex being the ultimate attention-it drives you crazy.
Either you are in love with him-or its the above. If it is love-then you really need to get this off your chest and tell him-yes it may rock your relationship if he's not interested-or it might not-but its better than living like this.
you say that you're not sexually attracted to him-but your deeply obsessed, amybe your replacing something in your life with him-did u have a good relationship with your parents? have u any brothers or sisters? I have heard of this with siblings too.
If you dont fancy him-or dont want to be with him-then you have to learn to let him go-hes a good friend so he wont be going anywhere, however if he cottons onto the fact that you feel like this-and he will eventually, he'll start to notice your behaviour-no matter how subtle, you risk alienating him. Let him see girls-why should he be on his own? make a move if you wish-but if you dont-let him have his life, and you should b looking out 4 yours more
see your other friends-go out and do things, have fun, study, work -whatever-and meet people.