There seem to be two kinds of horses: Those eager to get into a trailer and those eager to avoid getting in.
It seems the difference between these kinds of horses comes down to education: Those who want to get in have learned the trailer ride is acceptable and what will happen at the destination is likely to be pleasant. Those who don’t want to get in have also been educated.
In the world of horses, everyone seems to confuse training and motivation; thinking if a horse won’t do something, they need training. In fact, they often know exactly what to do, but need motivation. We have too many horse trainers and not enough horse motivators. Motivation matters.