I was going to say that my best friend tells me he loves me all the time, and that it's great, but that he is also my husband, but David beat me to it!
I truly feel like the difference between relationships that work, and those that don't, is the foundation of that relationship. The people who are truly best friends, and not just romantic partners, are the people who can make it through the rough times. Partially because there is more at stake than just a romantic relationship, but partially because true best friends understand each other in a way that people who are just romantically involved don't.
The romance part of a relationship is important, and it is great, but when it comes to the difficult times every relationship goes through, it is easy to walk away, because you can find the romantic part anywhere. That's not the hard part to find in someone else. Finding someone else you love as a best friend, and as a romantic partner, is finding something that is truly lasting.
What I'm trying to say in all that is that maybe you need to take another look at your best friend, and see if there could be something more there. Having a relationship with your best friend can be the greatest thing in the world, if it is what is right for both of you.