The idea is that most people see a relationship as an escalator where you move up somehow automatically. First you date, than you're going steady, then you move in together, marriage, kids etc. pp.
The hedonic treadmill is considered one of the reasons for this behavior. Many people are most happy at the beginning of a relationship, when they get less happy they try to do something to get back to the starting level.
Points here are:
1. The space of new things you can do with your relationships isn't one dimensional, you don't have to move in one direction to get happier.
2. Maybe you're just running in the hedonic treadmill and don't realize it, so take some time to evaluate your life before you do things that are hard to impossible to revert.