if you grow dependent on it to feel anything good in your life ("yes, 60 upvotes! thank God, finally some serotonin"), you use it as a tool to harm others or other use it to harm you, you spend so much time on it that you don't know how to cultivate relationships irl ) don't know how to act irl, period .... there are probably a lot of other reasons it'd be bad for someone but I can't think of any more