‘Vyar Elokim es kol asher asah vhenei tov miod’ - and Hashem saw all that he made and behold it was very good. By each of the days of creation it says it was ‘good’, why on the sixth day does it say ‘very good’?

Ramban----[free translation] The posuk is telling us that in general you find a little bad in every good, because bad is needed for the purpose of keeping the good. As we see with the creation of death that is bad but is needed to help promote goodness in life, since it causes people to do teshuvah.