random thought, nothing major. monica and ross seem to be the only ones out of everyone that has problems living with the other 4.
phoebe moved out of monica's apartment because monica was insane. despite living together for 5 years, there was often friction between monica and rachel regarding monica's neurotic apartment-related behaviour. even chandler and monica have problems in that regard. when phoebe and rachel's apartment is damaged by a fire, they move in with joey and monica. initially, they regard monica's as the more desirable apartment, which goes to the person that _didn't_ cause the fire. later they both realise they can't stand living with monica and want to live with joey.
when ross moves in with joey and chandler, he annoys the hell out of them and they can't wait to see him go (but fortunately are able to tolerate him based on their friendship). there are a few instances where monica and/or ross make references to growing up living in the same house - and they weren't exactly fond memories if i recall. there are also similar references to when ross and chandler lived together in college. rachel lives with ross for a time, but eventually moves out because (among other things) ross was censoring her phone messages.
so, the failures (or at least, the ones with issues): monica & ross monica & phoebe monica & rachel monica & chandler (ouch, all we're missing is joey, but does anyone seriously think that would work?) ross & monica (repeat) ross & chandler ross & (chandler & joey) ross & rachel
and the relative successes: joey & chandler joey & rachel phoebe & rachel (they did have a few conflicts, but they were resolved)
ok, so i suppose there aren't that many successes. perhaps i'm just grabbing random statistics and inferences and dumping them together without there really being much meaning, but, whatever. i get bored sometimes (read often).
but, all of the failures involve monica or ross, and none of the successes do. by my admittedly-flawed interpretation, anyway.
-brian