He's the one that took in Skywalker as a padawan. Therefore, it's his responsibility, or at least he thinks it is whether he likes it or not.

[Though Maul won't mention it to him if Usagi thinks it's a bad idea. He's been more reluctant as of late to add anything to Obi-Wan's plate.]

Something is always troubling that idiot. I don't think I've ever seen him smiling or even just at a state of rest. It's no excuse for being rude to you!