This is not true as stated. Please elaborate.
My bet is that you are thinking about D&L aminoacids, but even then it is not entirely true.
Give the guy a break, you know
what he means!
To the OP: This is one of the big questions, along with "How did life emerge?" etc.
Tha fact of the matter is, no one really knows why nature often favours a particular chiral arrangement, it is because it was
There are a number of theories to why it is this way. A popular line of reasoning goes (very vaguely) like so:
From a racemic mixture, a 'symmetry breaking event' occured, resulting in a slight excess of one isomer. Then, a process of 'chiral amplification' increased the relative amount of that isomer, and 'chiral tranmission' allows the transfer of this chirality from one set of molecule to another.
This ia largely speculative reasoning, although there is experimental evidence to back up these claims to a certain degree (no refs to hand I'm afraid, though I seem to remember a paper in nature from perhaps 2004-2005).
While this doesn't really answer your question, it might give you something to look into further should you be so inclined...