What keeps me going? Setting short and long term goals, then acheiving them. I've dropped 50 lbs in the past 6 months; I didn't get there by only telling myself "I want to be 50 lbs lighter in 6 months", and end it there. Setting that as your only goal is much too difficult to grasp, which leaves you floundering without a purpose. Have the goal you ultimately want, but set many small goals along the way that will ultimately lead you to your large goal. Breaking it down always helps, and acheiving each little goal is that much more motivation to keep at it.
Like right now, for example, i'm doing a clean bulk on an intermediate 5x5 program (If you have any interest in weight lifting, research 5x5, there is nothing that has the time tested and proven credentials of 5x5). At the beginning, I set my test weights slightly higher than the actual test weights I ended my last program with. I did this with the expectation of meeting my old records sooner, but ultimately having to reset sooner. I'm on my 6th week and am still setting records, and know that I have at least another 2 weeks in me. I had been expecting to reset by the 6th or 7th week. This is a long term goal that makes me happy. Going into the gym with my 5x5 routine all layed out, then killing all the sets for the day is a goal I acheive every time I leave the gym. If I start failing to acheive those goals, then I simply reset and take a new approach, so that the progress toward my goal isn't hindered.
The fact that I've increased my drive by nearly 50 yards since I started is a very nice bonus. And sure, I like looking good in the mirror and for the girls. That goes without saying. Ultimately, however, I lift for myself, the satisfying feeling of accomplishment after every session, the effect it has on my mood, my mental strength and endurance, and my overall outlook on life. If I take a week off, I notice the loss in all the aforementioned effects I experience from lifting. Having those effects in my life is what I really need to keep me going.