Adjusting to a parent dating somebody new is something that a lot of kids find difficult. This counts for both younger and older, but in many ways, it can be more difficult with those who have reached the adolescent stage. They are already dealing with the difficulties of that awkward transition to adulthood, and adding any major changes to home life on top of that can be enough to tip them over the edge.
That being said, unless this is a situation in which they are being mistreated, there comes a time when the rebellion has to end and a sense of normalcy established. For some, this takes longer than others, and for others still, it never happens at all. If it is bad enough, it can leave the partner in question with a dilemma: is this person that they are in a relationship with worth the baggage of a stepchild who hates them?