Doesn't letting go of all desire mean detaching yourself from meaningful relationships?

Yes I agree with you. Buddah had to have desire to want to teach the people about how to end the suffering. Without this desire he would have just continued to sit under the tree. Maybe it was needing his bowl filled that caused him to rise up and to continue on but isn't that also desire? to stop the hunger pains, to fill the belly, to get sustenance to travel on?