Why would God tell Jeremiah not to marry?

Then the word of the Lord came to me: “You must not marry and have sons or daughters in this place.” For this is what the Lord says about the sons and daughters born in this land and about the women who are their mothers and the men who are their fathers: “They will die of deadly diseases. They will not be mourned or buried but will be like dung lying on the ground. They will perish by sword and famine, and their dead bodies will become food for the birds and the wild animals.” Jeremiah 16:1-4
            God instituted marriage.  He believes in it.
            However, there are times when things are so bad, when God just sees so much pain ahead for the family, that He doesn’t see the use for people to invest in their future.
            This call to not marry was for Jeremiah the prophet.  As a prophet, people watched him closely.  If Jeremiah married, then people would think, “Oh, there’s hope.  Things are going to be okay.”  Jeremiah’s unmarried life showed that he believed the message he was telling the people—God’s wrath is coming and there’s hard times ahead, so don't plan for the future.
            This isn’t a command for all people today (be fruitful and multiply still applies), but was meant to show how bad things had gotten to people of that time.