Reactants refers to the fact that when we push people, they tend to push back. People want a sense of freedom or control. When we usurp that, they push back.

Endowment is the idea that people are attached to what they already do or have. They tend to be emotionally attached to things they’re doing already, and they’re unwilling to let them go. As marketers, we’re trying to get someone to buy into something new. If they already have something else they’ve been buying, then they already have something they’re attached to. So we’re not just trying to get them to do something—we’re also trying to get them to let go of something else, which is actually more challenging.