What other people are deciding to buy (i.e good reviews)
What is consistent with
your persona (ex. always
wants the newest tech,
brand loyalty)
Obligations or debts (ex. Your friend has
had you over for drinks several times
now, you want to return the favor and
offer them something as good or
better.)
Fear of loss (ex. An item is on sale and
you worry the price will go up if you
don't get it now)