Okay, I've been thinking about this one for a while, and I have something that I believe works for at least 7/8 words (I can't pin down the other) and the timing of the initial puzzle - something that was hinted at as being relevant. I believe that 'Fox', 'Pristine', 'Asteroidea' and 'Dawn' in particular work incredibly well under this theory, so let's put this out there as a possibility...
I think a Hot & Cold Word might be:
a word with a synonym related to Christmas. The reason the OP hinted 'It will be harder to solve this puzzle in the future' would therefore be that Christmas-related words would be more likely to come to mind during the festive season (when this question was posted).
Furthermore, these words would be called 'Hot & Cold Words' because depending on the hemisphere in which you celebrate the occasion it will fall either in summer (southern) or winter (northern), so the weather might be hot or cold!
The pattern among the given words would look like this:
Fox = VIXEN, one of Santa's reindeer.
Agreement = RECONCILIATION - in Christianity, an element of salvation brought about by the events of Jesus' birth, life and death. Alternatively, PEACE and HARMONY are nice fits here too.
Pristine = IMMACULATE - the 'Immaculate Conception' of the Virgin Mary.
Asteroidea = STAR, like the one which guided the Magi to Bethlehem.
Investigator = I am yet to decide on a truly appropriate synonym here - possibly something connected to the Magi?
Dawn = ADVENT, the period leading up to Christmas.
Conversation = COMMUNION - another Christian concept relating to remembering the life of Jesus.
Assistance = SERVICE - many religious services are held in the Christmas period.
The opposing words listed have little or no connection to this theme and have been chosen just for the similarity to the words in the left-hand column (although I appreciate that 'child-bearing' plays a pretty major part!).