Vanity is a big part of it for me. I don't want to look the way I did on my 20th birthday this year, on my 21st next year.
How I feel when I've lost weight is also a motivator. During sixth form, I lost nearly two stone. I didn't reach my goal weight, but I was certainly fitter than I had been, and felt a lot better for it.

Unfortunately, I then put on about 3 1/2 stone when I came to uni. Whoops
Clothes are another one for me. Last summer, when I was at my lowest weight after that weight loss, my parents paid for a dress for me to wear to my uncle's wedding. It's a gorgeous dress, flattered me in all the right places etc and made me feel brilliant, and seeing as I like to buy clothes mainly on a "cost-per-wear" basis, I'd like to fit into it again so I can wear it when I go out.
Another dress drama... I got a ticket to go to a ball at uni just before Easter, and so asked my parents to send up my prom dress to I could wear it. Of course, the prom dress doesn't fit anymore, which I found rather embarrassing to have to explain to my mum when she asked how the ball went (though obviously she could see for herself how much weight I've put on).
Ah well... I'm working towards losing the weight now, at least.
Another thing that motivates me is how great I feel after exercising. For example... I went on a walk earlier today. When I got back, I felt so good that a few hours later, I went for another walk.
