How and why did Nubians and Egyptians interact?

Nubia is Egypt's southern neighbor. The Nile continues up through Nubia into the interior of Africa. For Egypt to access all the wealth and goods of inner Africa they need to control the transport on the Nile through Nubia. Also Nubia was full of gold just waiting to be mined. So Egypt conquered Nubia, established forts and toll stations at the River's cataracts, and put the Nubian people to work mining the gold for them.