Why do Jews sit shiva?

There's a fundamental understanding that G-d gives and G-d takes away and in G-d's name we pray. That's what we say when we experience grief and when we are grieving we can't just continue. So the tradition creates ritual to honor what we're going through and to reassure us that there's a path from devastation to healing, and shiva is one of those pathways. In shiva, we stop. We don't do anything, and the community comes and takes care of us.