Shaking while lifting weights??

I have been working out for years now, and was wonder what causes me to shake when i'm lifting relatively heavy weight? For example i was incline dumbbell bench pressing, i started with 55 with out shaking then moved to 65. This is when i start to shake when i get to the top of my set. It's not really that heavy as i can do it 8 or 9 times with good form but i just can't stop my arms from shaking. I never have noticed many other people at the gym shake while lifting weights. Was just curious if any of you guys have the same experience or if there is a reason for it?