Of the many instruments that have evolved over the centuries in the music world, only one of them is faced with the challenging task of generating its own sound/pitch. When we think of instruments, we usually have a piano or trumpet in mind, in essence an instrument that needs to be played by someone or something. The human voice, however, as an instrument, has a very different set of rules that need to be followed.
Pitch production in singing starts with listening to the pitch in either the minds ear, or an external source. It is then processed in the brain to be turned into sound with the movement of air through the vocal folds. The instrument itself (the person singing) tries to create the sound heard as accurately as possible.
For singers, pitch and tuning cannot only be measured by hearing sound but predominantly by feeling the sound. Pitch production or pitch reproduction cannot be seen or touched as if playing a piano key or plucking a violin string. It is an abstract concept, neither touched nor seen, that allows the instrument to feel the vibrations moving through the instruments resonators. The constant changing shape of the vocal tract due to vowels in singing also makes accurate pitch tremendously difficult to achieve and maintain. It is a constant battle to keep the instrument aligned, the energy levels high and the pitch as centred as possible. When vocalising, pitch is in a constant fragile balance that can never be neglected, but eventually with experience and training the issue of pitch integrity becomes easier to maintain.
Among musicians pitch can be easily taken for granted, but for vocalists this should most definitely not be the case. As mentioned above maintaining pitch integrity is challenging, but with healthy vocal training it can become second nature. Having said that though, the pitch should always be nurtured, no matter how experienced the singer.