In general Germans place very high value on reliability. Colleagues who keep their word are considered to be treu (constant, faithful, loyal). A Ja from a reliable person means I really can count on that person.
Germans are particularly aware of the importance of this virtue in their culture. They expect commitments of any kind to be kept. Zuverlässigkeit ist Gold wert – reliability is as valuable as gold.
Ein Mann, ein Wort – one man (person), one word – is a sought-after character trait in Germany, in both the work environment and the private sphere. A quick, but not fully reliable, Ja is considered to be of low value.
Gesagt, getan – said and done. Germans expect words to be put into action. Versprochen ist versprochen und wird nicht gebrochen– promised is promised, and will not be broken. Germans are determined to keep their promises. Ein Ja ist ein Ja – a yes is a yes. Once said, it should be kept.