In certain societies, in order for a marriage to take place, the groom must purchase the bride from her father, in return for a certain number of camels. (I vaguely remember reading this somewhere, or perhaps heard it on the radio, so it must be true)
In others, the father is obliged to pay the groom to take his daughter away, again in livestock, i.e. camels.
One can only speculate as to why this is so. Possibly the daughters in the first tribe are more attractive than those in the second.
But the important thing is the central role played by the camel. It is hardly important in which direction the camel is travelling, after all.
Which leads naturally to the question of why this foul-tempered, evil-smelling, vicious quadruped has become the medium of exchange necessary for the continuance of the human race in these tribes?
Possibly it explains the preference for raiding instead, in which obtaining a wife does not require the involve of camels.