As the firetruck got closer, sal noticed that the pitch of the siren got higher. why did this happen?

The pitch of the siren of a fire truck appears to change as the truck passes us due to the Doppler Effect. The Doppler Effect is the change in frequency of a wave due to motion of the source or the receiver course. Although it sounds higher, the pitch actually does not change at all. 

Since the speed of sound in air is essentially fixed, the perceived pitch of a tone is related to the wavelength of the sound. The shorter the wavelength, the higher the pitch, and vice-versa. The wavelength can be conveniently considered to be the distance between successive 'crests' of the waves of pressure that constitute the sound wave. 

hope this helps!