according to kinetic molecular theory, everything that has even a smidgen of energy in it -- i.e. has a temperature greater than absolute zero -- is vibrating. by and large, macroscopic objects vibrate very slowly, so you're unlikely to notice them doing so. this vibration is usually referred to as natural frequency: the number of times an object oscillates (moves back and forth) in one second, measured in units of Hz.
physically speaking, everything in the universe has a natural frequency -- even you and me. metaphysically speaking, i would like to posit that everything also has a natural frequency on a spiritual, emotional, psychological, moral, philosophical level. in people, this is manifested as our personalities, codes of ethics and interests. it dictates what we are attracted to or what we are repulsed by, who we get along with and who we really can't stand, why candy tasted so awesome as a kid but has since lost its appeal 10 years later. it's all in the resonance.
have you seen the physics demonstration of two equivalent tuning forks affixed to sound chambers? ringing one fork results in the other ringing apparently on its own accord, without any visible connection (wires, etc) between the two. click here to watch.
this happens because tuning forks of equivalent frequencies resonate with each other (and anything else with a similar natural frequency). when a tuning fork is struck, the tines vibrate at their natural frequency, compressing the air in regular intervals, thus generating a sound at that frequency. when an equivalent tuning fork is held near, the air compressions (sound waves) push against this new fork's tines in regular intervals -- the same intervals, or frequency, as that of their origin.
if you impart a force, even a very small one, at the same frequency as an object's natural frequency, it will move, regardless of how big it is relative to the force. think of it like this: a big kid is sitting on a swing and wants his little sister, who is 1/3 his size, to push him really high. since it is physically impossible for her to do so with one push, she does the smart thing and gives little pushes every time her brother reaches the apex of the swing, just as gravity begins to pull him forward again. eventually, the swing will have accrued enough energy to make the boy happy. same thing with the tuning forks. little bursts of air pressure at the natural frequency of a metal tuning fork can set it in motion. even on something as large as a bridge, resonance can move the entire structure.
frequency can also be understood as wavelength; since everything has a frequency, everything has a wavelength, and is thus a wave. when waves meet, they combine together (superposition principle) by adding together. crests adding to crests makes bigger crests; troughs adding to troughs make deeper troughs, etc. this is called interference. resonance is an example of constructive interference, when everything lines up to make the wave amplitude greater. conversely, there is destructive interference, in which the waves add up to reduce the amplitude.
each person is a bundle of personality, experience, morals, emotions, and thus has a complex set of natural frequencies. things in the world also have frequencies that bombard us daily. some of these things resonate with us; we interpret them usually as, "i like that" or "that was pleasant" or "OMG i'm in love." strong negative feelings are also examples of resonance. the resultant amplitude of the wave interference indicates the intensity of these feelings, be it love or hate. of course there are also things and people that just seem to have no effect whatsoever on us; they are things that are so uninteresting to us that we are essentially blind to them. destructive interference is happening there.
as you grow and accumulate experience, your natural frequencies will inevitably change; the frequencies that hit you everyday will mold you into something different. think back to the tuning fork demonstration; one could prevent resonance by simply adding a mass to one of the tines of one of the forks (like a lump of plasticine). this happens to us too: see enough hypocrisy and you'll grow cynical; watch your mother get hacked up with a chainsaw when you're 3 years old and you'll turn into a serial killer devoid of emotion; spend a lot of time with someone and you'll pick up the same mannerisms, habits, preferences. by the same token, these 'masses' can be shed if you know where to look, restoring your original frequencies, for better or worse.
