Do you like the way your wife is kind to strangers or is especially
good with children?  Does she have good posture and impeccable
manners?  Is she frugal with the family income and creates great
meals?  Are there any other attributes you especially appreciate in
your wife?  Whether you like it or not, your in-laws may have
something to do with it.

Have you ever thanked your mother and father-in-law for some of the
great qualities they instilled in your wife?  Punctuality,
politeness, strong work ethic, honesty, neatness and humility are
just a few of the virtues her parents may have taught her.  And it
is time you thanked them for their part (no matter how small) in
molding your wife into the person you love.

This exercise if particularly helpful if you don't have a close
relationship with your in-laws.  It may help smooth over any
friction that has occurred between you and them.  All people
(including in-laws) have at least one good quality.  For you to let
them know that you notice and appreciate that quality will go a
long way in continuing a blissful marriage with their daughter.

A good time to share this with them would be your wife's birthday
or on your anniversary.  Your wife's parents were instrumental in
rearing and teaching their daughter since the day she was born and
each year on your anniversary you remember all the wonderful
qualities that made you want to marry her