Because it´s an addictive game. But it is not the game itself that produces addiction, but how well you say it´s the community.
This forum and the chat are not casual, they are the bait to have you caught. Many times is created in reality the false sensation of having friends and enemies, as happens in social networks. But it reaches to have you hooked with people who share your same interest in the game. You remain here for them and they remain for you. You will be supportive with the people who sympathize with you and competitive with whom you have no feeling.
It´s likely, as it has happened to me, that among all of people you really find a true friend, whether you get to know him personally or exchange private conversations outside the game. The issue is that if talk revolves around the game too, then you cannot let go.
So, if you have a real friend and you know that your friend really wants to leave the game or this game is not good either of the two, it's best to avoid conversations about this, talk about anything else (There are so many topics to talk about!), at least until you both feel that they are no longer addicted to the game, so can stay or leave the game whenever you want, without affecting the friendship between you.