It is most likely not because someone do not want to be your friend... its just that they might not know you well enough, or at all for that matter, or know enough about you to consider adding you as a friend to their friends list.
As another warrior recently mentioned adding friends may be construed as some sort of endorsement by those viewing your profile.
Also, if you use an anonymous moniker for your username you are much more likely to be rejected... especially if your profile offers no clue as to who you are.
I am happy to get friend requests but if I do not know you well enough or at all and reject your request do not take it personally... lets become friends, do business, and get to know eachother first.
Of course if we have not had much interaction but I know you from your activity here and it appears that you are trustworthy and respected I would probably add you even if we were not personally acquainted.
I really only see two benefits to the friends feature on the forum:
1. You can change settings so that only friends can communicate with you privately.
2. A friends list is a way of showing others who view your profile those whom you respect or interact with comfortably here on the forum and in business.