You become part of the antenna.

when you touch the aerial of your radio, you increase the lengh of the antenna by adding yourself to the circuit, this increases the signal strengh. this increases the number of decibels that your antenna, meaning you get a stronger signal. the stronger the signal, the louder the radio becomes.

Is JUST "Because Your Corporal Electricity Impedance"... When I was a child, always I Touch the Family Radio Tuner = to Pump Up The Volume... Nobody tell me this practice, but I Discover by myself = "My Self" ???... Sincerely Yours. lucy1771

All aerials are directional. There are always some directions from which the received signal is greater than from others. The polarization of the radio waves is also an important factor. For instance, a vertical aerial will not pick up horizontally polarized radio waves. Also, the physical position of the aerial may be important in an indoor environment. This is because multiple reflected waves can cancel each other out in certain places and enhance each other in others.

Because the human body is conductive, touching an aerial, or even moving around near it, will generally change its directionality and/or polarization and can change its effective position relative to signal nulls. This can dramatically change the received signal strength.

All of the above assumes that the aerial length is optimum, i.e. a quarter wave rod aerial or half wave dipole. If the aerial is the wrong length for the frequency, touching it may change its effective length favorably and increase the received signal strength.