In chemistry there is something called the activation of energy. It means to get a reaction started you have to add quite a bit of energy to it. Lighting a match to start newspaper on fire to start wood on fire is an example.

In the curve below notice that the line on the left is low, lower energy. Then enough energy is added to get over the hump and the line on the right is higher, higher energy. Getting over the hump is our problem. Notice that if a catalyst is used the hump is much lower. In our case maybe that is something we are very interested in already, or say an antidepressant, or maybe things we learned in therapy that helps us with motivation. Say the line on the left is a normal person. Well our line on the left is much lower to begin with so it is even more energy we have to add to get going.