Genesis 9:7

"As for you, be fruitful and increase in number; multiply on the earth and increase upon it."  Genesis 9:7

Here, God is echoing the same instructions he gave Adam and Eve in Genesis 1:28 when he first created mankind and blessed them with life.  He wants humans to thrive and expand.  Despite his earlier punishment, God still loves mankind and wants good for us.

BibleRef points out that this is following God's harsh stance against killing.  Not only are humans made in his image (as reiterated in verse 6), but he wants to stop the kinds of violence that ruined the world before the flood.

He's not still holding a grudge, saying "Okay, you survived my punishment, but lets see if you make the final cut."  No, God is telling them that they have a fresh start, the world's slate has been wiped clean, now go forth and live.  In the same way, we make mistakes and sin.  Then we repent and ask for forgiveness and God (through Jesus's sacrifice on the cross), gives us that forgiveness.  Our slate is wiped clean, now go forth and live.