Leafs make this trade because:

In this scenario, I feel as if Seattle would take Kerfoot from Toronto and Graves from Colorado. Now instead of giving Seattle two players that would benefit their team, Toronto and Colorado swap those players to help each other out and force Seattle to pick a lesser player. Obviously Colorado is familiar with Kerfoot and that is where he played his best hockey. I feel as if this is a win-win for Toronto and Colorado and Seattle gets the worse end of it.