Gay men, straight men, and everybody else will tell you that the pain of falling for a best friend, only to have that feeling remain unreciprocated, makes this is a less than palatable trope. Good grief, a horrible movie got made about it. And my empathy on this subject has a seriously short shelf life: see The Horrific Trope of Unrequited Love.
However, I think critics of this trope in M/M romances miss two important elements: one, friendship growing organically into love; two, unexpected and unforeseen love arriving in the protagonist's life.
"Unexpected" and "unforeseen" are the passwords here. With Eponine, for instance, the possibility of Marius being interested lingers, however hopeless. But with straight and gay best friends, the possibility is supposedly nil.
Hence the delight in having love creep up on one: here it is, and sure there is effort involved, but at least you didn't have to try to force it! Or run after it! Or plead for it!
But I think another reason is the idea of the unexpected. Social expectations supposedly set love aside. And yet it arrives! There it is!
Work, self-knowledge, misunderstandings, forgiveness still enter the picture. But for once, isn't it nice that it came about without so much exhaustive peacocking?