Just as Obi-Wan realizes that Anakin hasn't seen him, and before he can do more than open his mouth in warning, the young man's swift pace has them bouncing off each other. Obi-Wan is barely able to catch himself from falling on his ass, and once he steadies and rights himself, he starts laughing.

They're both far too tense, honestly. While of course there is a war raging, it's not good to be so keyed-up all the time; a little humor injected into things to lighten up from time to time is good for them. Especially if it's at their own expense.