Three things motivate me:

1. The fact that I pay a lot for the damn gym fees.
2. I feel weird, agitated even, if I don't work out after some time.
3. I'm health-conscious, so exercising and eating right and leading a good life motivates me to lift 20kgs above my head.

Mind you, there are some days I think to myself, "Why the hell am I doing here? Why am I lifting 70kgs off the damn floor?" Then I shut up, do it and let the adrenaline rush take over me. One of the best feelings, ever.