Enthusiasm begins badly, and the text says so without qualification. Legge's translation of the first line shows its subject proclaiming his pleasure and satisfaction, and pronounces evil. No condition is attached and no remedy is offered, which is unusual in a hexagram whose other lines mostly leave a way out. The fault is not the pleasure. It is the proclaiming, and specifically the fact that the proclaiming happens at the very bottom, before anything has been accomplished that would justify it.
The position explains the severity. A yielding stroke stands in an odd place at the foot of the lower trigram Kun, incorrectly placed by the positional rule and as far as it is possible to be from the single strong line at the fourth position that supplies all the energy in the figure. Classical commentary reads the first line as connected to that strong line, and treats the boast as a person trading on an association with someone powerful rather than on anything of their own.
Read against the hexagram as a whole, this line supplies the pattern Enthusiasm keeps testing: energy is real, and it belongs to whoever generates it rather than to whoever is nearest it. Thunder rising out of the earth is the figure the hexagram uses, and thunder announced in advance is only noise. Everything the remaining five lines describe, steadiness, delay, suspicion and late correction, is a variation on the error committed here first.