1. In the past (over 100,000 years ago), humanity was fighting a war with the forerunner. This came about when the forerunner attacked humans who were spreading into their territory. Humanity was doing this because they were running from the flood. But the forerunner didn't know this and took it as an act of war and obliterated the hell out of humanity.
It wasn't until after the fact that the forerunner discovered the flood, and that humanity was running from them. They were like "aww s***". So for one, the forerunner owed humanity something for mistakenly deciding to obliterate their race.
Secondly, they discovered that humanity found a cure for the flood, and they were willing to sacrifice however many of themselves to do it. Not only were humanity willing to do what even the forerunner could not, but they were able to stop the flood once, they could theoretically do it again if ever required.
[Edited on 06.21.2011 11:47 AM PDT]