Why is the islmic prayer (salat) obligatory?

standing praying is part of the contract between a slave worshiper and his God. if it's broken you have broken one of the important pillar and the relationship between you and God. yes it's a sin indeed. there are required prayers which are the five times praying, and there is voluntary such are "nawafil" prayers. there is required charity and optional one, there is a required fast and optional one. with all the goodness that God has given us, we can't pray five times a day to thank him for it?