I know everything has a natural frequency that is a intrinsic property to the body in question. For example, a string vibrating has the largest amplitudes when it's vibrating near it's natural frequency, and the same thing happens with other things too (like the Tacoma Bridge). Natural frequencies also play a important role in control systems. Systems have natural frequencies that you have to take into account when you are designing controllers for them. While I understand reasonably well how natural frequencies "work", I can't really grasp where do they come from. I mean, is there a reason to why every body has a natural frequency? And why systems have large amplitude oscillations when in resonance?