Our bodies are very tightly regulated when it comes to maintaining homeostasis, things like temperature, blood pressure, ph, blood sugar etc. are all closely monitored.
When you exercise you get a temporary disruption of our normal internal environment which then recovers to normal post exercise. Your body adapts to this response by getting 'fitter', such as increasing the ability for your heart to pump the blood so the next time it won't be perceived by such a stress.
This is, presumably why some people don't like exercise, it's essentially another stress on the body and must be performed to a certain 'discomfort' to elicit a response.
This doesn't mean you have to run sled pushes until it changes your religion, but nor is it likely to do much for you if you don't move out of your comfort zone to some degree.