Be careful with that feeling and don't underfuel, or at least keep it at "sane" levels. I feel pretty amazing and full after 62km/2700mD+ XCMs as well, as an extreme example... which is at least partially due to the immune system (and resp. inflammation/etc) being suppressed. Long, light/moderate efforts without adequate food intake and rest can lead to the same thing.
What are XCMs? 2700mD? Why would the immune system being suppressed make you feel good? I’m so confused
Stress hormones. Read up on cortisol's effects. XCM - cross country marathon (MTB race). 2700mD+ - 2.7km of vertical gain. A reduction of inflammation is a general effect of anti-inflammatory drugs, which tend to make you feel better. There's a LOT more than that (i.e. say, all the things that fall under the umbrella of "runner's high"), but the TL:DR is that significant physical activity / energy expenditure, combined with a lack of proper rest & nutrition leads to long-term undesirable effects that can definitely be masked and/or disregarded.
I fugure XCM must be cross country marathon, where 62km is the distance. But what is mD? Difference in elevation in meters?
I have no idea what you wrote and you shouldn't assume people know some niece jargon.