Why do you train? I don't know. I just do.
Just train. Treat it like a job or an appointment. Whatever your schedule is, the secret to improvement is to just be consistent. Of course, life happens and things happen out of your control and sometimes you won't be able to make it to class because there are some things in life that are more important than your jiu-jitsu training...but don't skip class just because you don't feel like going. Don't skip class because you're lazy and feel down. Go train. Usually you'll feel better after you train. Training can be wonderfully escapist. When you're fighting for your life or just to survive or trying to learn something new, you tend to forget about all the other things that happened during your crappy day. You tend to have a very specific focus when you are being "threatened" or "attacked" (i.e. training). When you are defending against someone trying to choke you or hyperextend your arm, your tend to get very focused and the rest of the world falls away.