There are lots of reasons why people may not offer rematches:
a) They got tired and need a break
b) They have a call incoming/have to leave
c) They decided to switch to a different time control
d) They need to analyze the game first
e) They didn't like your playing style/manners
f) You rating is too low to play you again
g) They want to play only White/Black.
And so on. Hence, don't be too hard on people not giving a rematch. It's not like each of them is a coward
h) They feel as though their rating is too high and are worried they might lose points
i) They feel as though your rating is too low and are worried they might lose points
j) They got lucky in previous game and want to "end on a high"
k) They got lucky in previous game and prefer their opponent to stew in their loss
l) They played one game to find out whether you are over-rated and stop if that is not true
m) They really really feel out-matched by you and don't want to just throw away points
Or they need to drop a bomb in the toilet asap.
Check this out: http://www.youtube.com/watch?v=4NxaC3jEGkY