I agree with the first part, inasmuch as volatility in one's personal life affects one's professional conduct and ability. The two cannot be entirely separated, but there are definitely varying scales of inter-connectivity.

Common sense and judgement fluctuate. Humans, you know, we're prone to entirely unpredictable moments of emotional weakness. Maybe we're hungry, maybe we're tired, maybe the missus or the kids have been sick, maybe we just don't want to face leg-day after 12-hours of strategising. As per the attack and defense argument: A defender only needs to be wrong once.