Shell : Why if-condition always true

In the following example, I find that the if [ $aaa==x ] condition is always true.

for aaa in {x,y} 
do 
echo ---------
echo $aaa;
if [ $aaa==x ]
then echo this is x
elif [ $aaa==y ]
then echo this is y
fi
echo $aaa;
done;

That is, I always get the output as :

---------
x
this is x
x
---------
y
this is x
y

Even if I replace == with =, the problem remains. Why? Thank you all for helping me!