-You want to stay in school.
-You’re afraid of the job market.
-You don’t know what else to do.
-You’re really smart and do well in school, so you should prove that by going as far as you can go.
-A graduate degree is required for the career path you’re aiming for, and you’ve researched it all thoroughly, including talking to people who hold the kind of job you want.
-You have specific research interests: there are significant questions that you want to answer, that with a little training you will be capable of answering, and that you can do at least as well, if not better, than anyone else at answering these questions.
(Note: This is a good reason for grad school with all other things being equal. However, given the current state of the academic/research job market and research funding, you must carefully research your specific field to rationally assess the chances that you’ll be able to get into a grad program and find a research position afterward)
-You have the means to pay for a program without debt, and you’re really interested in a given subject.