Why does the water we drink in summer taste cooler and the water we drink in winter taste warmer?

I suspect the answer to your question is a bit subjective.
 It depends a bit on where your water comes from.  Where I live, the water is pumped in
from a lake, so the water tends to be the temperature of the lake water.  During the
winter, that means it is fairly cold.  The temperature under the ground, where the water
travels, is warmer than the water itself (generally) but in the winter the water is not
warm by any stretch of the imagination.  The warmer water your are drinking may just
seem warmer by comparison because the first bit of it has been sitting in pipes inside
your heated house.  It may also be that because it is so cold outside if you take a
drink of cold water after coming inside out of the snow it will taste warmer by
comparison.

As for the summer, the same rules apply.  The
water from the lake is generally cooler than the air (maybe by 20-30 degrees depending
on where you live) and it travels underground, where the hot outdoor temperatures don't
reach as well.  It therefore comes out of the tap much cooler than the day, but the same
relativity applies: if you come in from a hot day and drink from the tap it is going to
TASTE much cooler.