Why do objects have internal energy?

[Vectronix]

From what I understand, the molecules/atoms that make up an object are in constant, random motion. Do electromagnetic forces cause this motion or what is the cause of it? If so, why would net electromagnetic forces exist between "neutral" atoms?

 

[mfb]

Do electromagnetic forces cause this motion or what is the cause of it?

The objects have some energy - and they just keep that (conservation of energy). You would have to create something at absolute zero temperature to avoid that, and you can't do that.

If so, why would net electromagnetic forces exist between "neutral" atoms?

They still have internal charges, and if their distance is of the same scale as their diameters these matter. In addition, the Pauli exclusion principle prevents the electron shells from overlapping too much.

 

[sophiecentaur]

Do electromagnetic forces cause this motion

They are the reason that an object doesn't"t just fly apart, due to the motion.
One way you could look upon the motion is that it's the share of the Energy that was created when the Universe started. That Energy has spread everywhere.