Why do I feel so much about the holy Gayatri mantra?

One reason is that, despite the great variety in the various religions, sects and practices that are together called Hinduism, and despite the great conflict between many of them, they all continue to pay homage to this short, simple mantram. Probably it's the one, and only one point in which the various sects of Hinduism agree! To the greatness of this exalted mantra!

Another reason is that, despite the vicissitudes over several millenia, it has remained part of the glorious tradition of Sandhya Vandanam that is an integral part of ancient Vedic culture, medieval Puranic and Tantric culture, as well as of today's Hindu culture.

It finds mention in the Vedas, which even conservative estimates place between 3000-2000 BCE, and it finds mention in the Upanishads(circa 1000 - 500 BCE), as well as in the teachings of lord Buddha (6th Cent BCE), in the Grihya, Srouta Sutras of Apastamba, Baudhayana et al (8th Cent BCE), in the Manu, Yajnavalkya and many other Smriti works (uncertain date, but earlier than 4th-2nd BCE) as well as in the works of Kalidasa (circa 1st Cent BCE). So does it find mention with great vigour in the works of Banabhatta (7th Cent CE) as well as in that of Thyagaraja (17th Cent CE). And it is still being recited by countless people throughout the world. In this context, the efforts of Swami Vivekananda, Saint Dayananda Saraswati(of Arya Samaj) and of Pandit Sriram Sharma Acharya(All World Gayatri Parivar) in popularizing this holy mantra among people belonging to all religions, all castes, needs special mention.