"However, some scientists question the mechanisms at work, their research possibly demonstrating the high comes from completing a challenge rather than as a result of exertion."

This is absolute bullshit. How much the high you receive after working out is specifically a product of endorphins or not might be up for debate, but there are many components to this high, and they are physical, not some amorphous "I finished, give me a happyface sticker!"

Think of this response in two ways: short term and long term. Logically, when you complete a ton of exertion, you are firing on all cylinders, producing endorphins, testosterone, HGH, neurotransmitters like seratonin and a whole host of other goodies. The increased bloodflow to the nooks and crannies of your body and brain alone no doubt has a positive impact on your sense of well-being. Heck, exercise is neceessary to shake up the synovial fluid in your joints to keep you well lubricated.

Then there is long term. That sluggishness that you feel when you are off the wagon also has a ton of components, perhaps chief among them hormonal. Fat accumulates, which lowers your body's production of growth hormone and testosterone, increases your production of cortisol, causes insulin insensitivity and jacks up your production of estrogen. A big part of the clinical description of "metabolic syndrome" (a precursor to Type II diabetes and heart disease) mirrors the sluggish feeling you describe.

A fundamental: dudes want less estrogen and more testosterone. It is a key ratio and set of levels to make a guy clear-headed, creative, aggressive, intelligent and happy. Exercise (specifically fitness) helps maintain these ratios in the face of middle age, which naturally starts degrading and throwing them out of whack.

Beyond that, the simple fact that doing no exercise fails to circulate your blood and oxygenate tissue has pretty bad implications for how you feel.

Exercise is nature's antidepressant. It's addictive, but there are worse things to be addicted to.

Endorphin release is a real thing, and as with a lot of things about the human body, it is a built in mechanism to cope with an external reality.

Physical effort can be fatiguing- imagine you are Early Man, trekking across the savannah, looking for that next meal. Endorphins make him feel better, help him cope with exhaustion, and thus more likely and able to continue that physical effort when necessary.