For me, the relationships are always deeper because the people involved know so much more about each other. It also seems as if the relationships have a better chance of weathering the tests of time - after all their friendship has lasted through its own tests. They are not just learning surface things about each other (they know those things already) they are learning to look below the surface. Finally, it seems more like love, and less like infatuation.
Being that it's one of my favorites, when I found this post & article I thought I'd share bits of them, maybe they can add some more reasons to like this trope :)
Beyond the book: Friend to lover trope by Taryn Kincade.
MR KNIGHTLEYEmma! You want our friendship to remain the same as it has always been, but I cannot desire that.
EMMABut why? I know I make mistakes, but had you been here the last few days you would have seen how I have tried to change! Please tell me I am your friend.
MR KNIGHTLEYI do not wish to call you my friend, because I hoped to call you something infinitely more dear.
The trope is as tried and true as the characters are to each other (at least until some annoying third wheel attempts to come between them and threaten a relationship as old and comfortable as a faded sweatshirt). They know they have each other's backs. ...
The conceit is particularly delicious for us, the reader or movie-goer, because we know something the hero and heroine don't: that while they are confiding in each other as friends, either bemoaning their lack of a sex life or dishing the deets of their relationship with someone else (someone obviously, hideously wrong) - they are absolutely, positively, awesomely right together. We are rooting for them both. ...
One of the things that's so tantalizing and intriguing for us, the reader or viewer, about the "friends to lovers" premise is watching the way old friends go about discovering each other anew, in a more highly-charged way. We, of course, feel a little smug and superior because we knew it all along! Don't you think that's true? We delight in watching our hero and heroine fall back on the comfortable and familiar, the things they were in "like" with all the time- even as that same crooked smile is suddenly worthy of notice and sets them ablaze. ...
And from the comments:
"I love the friends to lovers thing, it’s about people seeing each other properly for the first tiem – out of the box they have consigned them too. I think that’s why I like boss and secretary type books too. It’s that sudden twist in a familiar relationship that changes everything."
"I think one aspect of why I like this trope so much is because the two friends already have a high degree of trust between them. So often in romance (and romantic suspense, which I really, really like) a fair bit of the novel has to show the building of trust. With friends-to-lovers the trust is there, and it presents a different challenge — not to betray the trust, but to add hope and (romantic) love and take the relationship somewhere new."
Friends who become lovers - The Telegraph
The reality is that when friends become lovers the shift in their emotions is usually gradual. In fact, it's so subtle they don't even notice it's happening until the moment a kindly hug becomes loaded with intention. Mo Kurimbokus, a relationship counsellor, says, 'Think of it like foreplay. All the time you're being friends, you're learning about each other. Subconsciously you're deciding whether you can take it further, from a friendship on to a more emotional and sexual level.' ...
[from a real life example] Sophie and Simon have now been married seven years and have just had their first child, Maggie Mae. 'We are incredibly close,' says Sophie. 'I truly believe that because we were friends first our relationship has always functioned on a much deeper level, which is actually quite rare among couples. It's difficult for me to define why it switched focus that day of the wedding, but, looking back, I think Simon was starting to get under my skin. I'd go on dates with other men and find myself thinking about him, and once I joked that we should get together. I suppose I was testing the water to judge his reaction. Deep down I knew he already liked me when I made my move. We were sitting on the bed in his parents' spare room when he kissed me for the first time. If I'm honest it felt so familiar, and it wasn't a fire-in-the-stomach thing, but it made me very happy. All day I couldn't stop thinking what an amazing person this quiet man had become.'
If there's one thing the experts seem to agree on it's the healthy survival rates of many friend-to-lover relationships. Ray Pahl says, 'Friendship is often the basis for a deeper kind of love, one that tends to be more long-term.' While Kurimbokus adds, 'When you're friends first, there are so many qualities you're ticking off along the way. If it all adds up, then you've got a real fighting chance.'