Well based upon being your best firned, the person must be very close to you. They know beforehand your strengths, weaknesses, life, history, and have probably shared many wonderful memories with you. Based upon this, the outlook of being in a relationship with your best friend is seemingly easier, since there is no fear of the unknown, and you generally like or even love this person. Once physical attraction boundaries are broken and you are emotionally and sexually driven to be with that person, it is very easy for them to turn into your lover. I mean, you already know the person, and you can see what a great match they are for you. You can also see how you work out in an interactive sense, so any arguments could be easily handled. So again, yes, it is very easy.
This is a man speaking from experience. :)