well..let`s say it may ENCOURAGE self pity, not really CAUSE it.

for instace:

i call you on the phone and say:
"you don`t know my life is such a mess..i am depressed...etc."

and you say:
"ho dear i am sorry. i also some times feel like that."

and then i keep talking...and talking about how bad my life is...and especially when you agree and say "yes i see"
noding to everything..it`s like..i may repeat and go ON INTO my suffering, recreating everyhting in my mind even stronger (and beliveing the situations IS that bad). and thus the negative words flow and repeat in different phrasing....and i call you all the time to complain and get used to it and think everyhting is so bad....
and get addicted to getting attention by complaining...

that is what i mean more or less.
i don`t say there is NO place for sympathy!
i just think that we need it maybe only in the very first steps of healing...."let it all out" but not much more, because it makes add more drama to it.

you know like you are angry and overwhelmd and say everything in Too Bing words.... and then bause the listener agrees with you pities you and says he understand you, or maybe get frustated himself (if there is a person who did somehting bad to you) thus you go into a lot of negative thinking...and it makes you feel that the situation is worse...

and then you may ACT as if it really is and thus you are actually MAKING it worse.

now, I DON`T SAY that sympathy is always causing all this.
but i think that this is the direciton where it`s going. sort of sitting and crying instead of simply solving the problem.