A whole load of madness, that's what. 
For some, the madness is of the good kind, the magical kind. And there's no question as to why that might be. 

Being best friends - or even close friends – involves by default a certain level of trust and comfort. We're talking about a person you spend the majority of your time without the uneasy awkwardness of having to appear admirable. You know all about their best (and lamest) jokes, you know what they're going to order at the restaurant, you know exactly how mean they can get when the going gets tough. And they know just as much about you and possibly more. Being close with a person on that level where you enjoy a person's company for qualities they project without being self conscious means that you've come to admire that person for who they really are. And the comfort and respect that comes from that kind of interaction is unparalleled. 

Where there is comfort, there's also fun. They wouldn't be your best friend if you didn't enjoy time spent with them as you would at a party, even after a tiring day. They're the person who can make you laugh at the worst of times. As much as we love spending time with friends, having a significant other you can act mentally deranged with is the best kind of happiness imaginable. 
“I can still be my weird self around her, which would have been impossible if it were someone else,” says Ashraful Ashik, best friend-turned-boyfriend to his girlfriend of 6 years. 

His girlfriend Audhora elaborates, “I never have to worry about how I look. I can say anything without being judged. We have very different interests and yet we are very similar. We are a codependent couple. We're Marshall and Lily!” 
While it all seems, and feels, like a magical joyride for the fortunate ones, falling in love with a friend may also turn into a nightmare. 
“It can complicate things,” says Nazmul Abedin when asked about how falling in love with a best friend can be inadvisable. “Sometimes it may not be mutual and if it isn't, there's a very good chance you're going to lose someone important from your life. And if it is mutual, being compatible as best friends won't necessarily mean you'll be compatible as a couple.” 

Whatever the reasons, failed friendship-turned-relationships give rise to a bitterness that by far outweighs that of a regular break up. It brings to mind something I read in a story once, “What if they fall out of love? And worse, what if one of them falls out of love first?” It just goes on to remind us of how the risks involved are somehow heavier. The ensuing resentment or awkwardness takes a long time to overcome and even goes on to hamper the dynamics in a group of friends.

Whatever the consequences, friendships turning into love stories have a certain weight to them that makes them extra special. They can turn into something beautiful and permanent or something beautiful and short-lived. The only thing to keep in mind is how much more sensitive things can be when embarking on such a relationship. 

As for my personal opinion, all it brings to my mind are infuriated fights breaking out into laughter after 30 seconds of eye contact, and two people dancing on Skype at 2 am because they were bored.