六 is thought to have first pictured a simple hut or shelter — a roof with two supports. It was then borrowed for its sound to mean 'six', and the original 'shelter' meaning faded away. The roof-like shape remains.
Six is a lucky number: it sounds like 流 (liú, 'flow') and 溜 (smooth), giving the blessing 六六大顺 ('six-six, everything goes smoothly'). Dates and phone numbers with 6 are prized. 6, 8, and 9 are the three numbers Chinese actively seek out.
A little roof on two legs — a shelter borrowed to mean SIX.
六六大顺 — 'six-six, all smooth sailing' — is a common blessing, because 六 sounds like 流 (to flow). 6 is one of China's lucky numbers.