East is considered to be the cause of all kinds of knowledge, auspicious and prosperity.
The Mahabharata has a detailed significance of the East direction and thus, why must we face East.
It gives all the major incidents associated with the East, and thus why it is considered the most auspicious.
Devarishi Narada narrates it as follows -
Udyog Parva, Mahabharata
O Galava, commanded I have been by God, who is the cause of all
knowledge. I ask you, towards which quarter shall I first take you to
see what lie there? The eastern, the southern, the western, or the
northern, towards which, O best of regenerate persons, shall I go, O
Galava? That quarter towards which Surya the illuminator of the
universe first rises; where, at eve, the Sadhyas engage in their
ascetic austerities; where that Intelligence, which pervades the whole
universe first springs; where the two eyes of Dharma, as well as he
himself, are stationed; where the clarified butter first poured in
sacrifice subsequently flowed all around; that quarter,
O best of all
regenerate persons, East is the gate of Day and Time. There the daughters
of Daksha, in primeval times, gave birth to their children. There the
sons of Kasyapa first multiplied.
That quarter is the source of all the prosperity of the gods, for it
was there that Sakra was first anointed as the king of the celestials.
It was there, O regenerate Rishi, that both Indra and the gods
underwent their ascetic penances. It is for this, O Brahmana, that
this quarter is called Purva (the first). And because in the earliest
of times this quarter was overspread by the Suras, it is for this that
it is called Purva. The gods, desirous of prosperity, performed all
their religious ceremonies here.
It was here that the divine Creator
of the universe first sang the Vedas. It was here that the Gayatri was
first preached by Surya unto the reciters of that sacred hymn. It was
here, O best of Brahmanas, that the Yajurvedas were delivered by
Surya (unto Yajnavalkya). It was here that the Soma juice, sanctified
by boons, was first drunk in sacrifices by Suras. It was here that the
Homa-fires, (gratified by mantras), first drank articles of cognate
It was here that Varuna first repaired to the nether regions, and
attained to all his prosperity. It was here, O bull among the
twice-born, that the birth, growth, and death of the ancient Vasishtha
took place. Here first grew the hundred different branches of Om
(different Vedas and Shrutis)!
It was here that the smoke-eating Munis are the smoke of sacrificial
fires. It was in that region that myriads of boars and other animals
were killed by Sakra and offered as sacrificial portions unto the
gods. It is here that the thousand-rayed sun, arising, consumes, out
of ire, all those that are wicked and ungrateful among men and the
This (East direction) is the gate of the three worlds. This is the path of
heaven and felicity. This quarter is called Purva (east).
And since East is the primordial source of all major incidents and scriptures within Hinduism, and is said to bestow prosperity and auspicious, thus East is the direction which is to be resorted to for any new task or daily start of the work-routine or day.