Generally dipole as the name suggests has two poles and the feed is in the center. The current distribution over a dipole is a sinusoidal wave with a maximum at middle, so feeding at the center provides minimum resistance. At this point voltage is minimum. So if you consider half wavelength of sinusoid accross the complete dipole with zero at the center you will find maxima for voltages (quarter of wavelength from center on either side) but with opposite signs. Thus half a wavelength is necessary to have to optimum response from a dipole. You can always use multiple of half wavelength structure to get the same response.
Regards
Safwat