近墨者黑

“One that nears an ink stick turns black.”

There goes why it is important to pick one's friend cautiously.  It saddens me to see a good-natured acquaintance of mine assume bad traits gradually from his/her friends.  Not going into details (because this particular case is in the Korean hemisphere of my life and would require more background explanation than the case itself), but I really hope all of my friends who see this would get the message.  *sigh*