One nice thing about working out is finding out how strong I am, and how far I am able to push myself physically.
A few weeks ago, my trainer had me do an exercise called the "plank." Basically the point is to hold yourself in what is close to push-up position, only your weight will rest on your forearms rather than your hands. Holding yourself up like that is supposed to strengthen your "core" muscles--i.e., your abs an lower back. The first time I did it--I won't lie--it hurt tremendously. Not just on my back but my whole body seemed to ache at the moment I began. But I kept myself up, exceeding the time my trainer had suggested I try, impressing not just her but also myself. A couple weeks later when I tried again, I had more than doubled my time. It impressed me because just the very first time I did it I thought I wouldn't be able to hold out for more than 4-5 seconds.
More recently, my trainer had set me on a machine for doing a kind of chest press. After completing my first set, she laughed--apparently I was the only one of her clients who was able to use this machine at all on the first try (the others were unable to do so, even on the lightest setting!)
And I had thought I was someone who was not particularly strong at all, at least not physically. But sometimes it's nice to surprise myself, finding that I'm stronger than I thought I was. It motivates me to keep working, and to try and get stronger. I feel like I can always push myself just a little bit further.