As to the honeymoon gone:
The short answer is: there is an increased number of dopamine receptors because of a deficit of T (lack of strong dopamine signaling). Once T levels are up - a high. Then the body reduces the dopamine receptors to compensate for the increased dopamine signaling caused by increased T.
Here is some text from Dr. Mariano:
Because there is a larger number of dopamine receptors from the dopamine signaling deficit caused by the loss of testosterone, there is dopamine supersensitivity to the surge of dopamine signaling that accompanies the increase in testosterone with replacement. This can cause a high - with heightened sex drive, alertness. and an elevated mood.
Testosterone would also free up thyroid hormone by reducing thyroid binding globulin, reversing estrogen's effects, improving function from this angle. This would improve energy
Testosterone would then reduce excessive norepinephrine signaling, which as it comes more in normal physiologic strength, helps dopamine in providing a higher level of libido, sex drive, and an emotional high.
The testosterone to estrogen ratio would improve, reducing effects of excess estrogen. Insulin signaling is reduced. The body becomes less in an inflammatory state.
The person feels better, if not feels a high from the initial treatment with testosterone.
Over time, however, with increased dopamine signaling, dopamine receptor production is reduced back to a normal amount. Dopamine, as the reward signal, the feel good signal, can't be elevated for a prolonged period of time excessively, without problems occurring. It no longer becomes a reward signal if it is elevated for a prolonged period of time. Tolerance, through receptor reduction, occurs.
After the initial high, other problems also occur.
Exogenous testosterone suppresses testicular thyroid releasing hormone production. This reduces thyroid hormone production, undoing the initial increase in free thyroid hormone that testosterone caused. If there is hypothyroidism in the first place, this exacerbates that problem.
If there are other neurotransmitter, hormone, cytokine signaling problems or metabolic-nutritional problems outside of hypogonadism, these may complicate or undo what testosterone initially did.
If the man aromatizes testosterone to estrogen excessively, problems with excessive estrogen occur. If aromatization is not enough, then problems with too little estrogen occur. In either case, sex drive is impaired.
Thus, the hypogonadal man returns to Earth. And the initial high is lost.