My thought is this: Points of attraction are also points of comparison. The things that attract you to someone are things you can examine in yourself and find wanting. The things that keep you up at night with longing may be the things that keep you up at night with insecurity.
This is true for straight couples as well, obviously — if you're attracted to someone's smarts or accomplishments or charisma, anyone of any sex or gender can hold themselves up for comparison and get down on themselves. The world is full of people who think their loves outshine them intellectually or socially, whether or not that's true. But body-insecurity is special, I think — it's particularly pernicious, particularly resistant to mollification. A straight woman won't look at a potential love interest and see a one-to-one comparison between waist size, bust size, hair curliness / straightness / color. Even if you think your boyfriend's hair is prettier than yours (*ahem*), hair doesn't play the same societal role for men and women, which makes it an innately uneven comparison. A straight man won't compare his penis size to his girlfriend's...usually. But surely this sort of thing must be a common occurrence in same-sex couples. And insecurity must sometimes transform into resentment, the way it does; attraction must sometimes evolve into sour grapes. "She's so pretty!" -> "She's prettier than me." -> "Stupid pretty jerkface, thinks she's so special."
Does this hold water? Or am I flapping my gums like an ignoramus? Do the walls between self-perception and other-perception hold regardless of body type? One mitigating factor may be gender roles — what's femme-attractive and what's butch-attractive may not usually have a lot of overlap, for instance. But what's plausible and what's real don't always overlap either, so responses are appreciated.