Because I just want them to be happy.
Obi-Wan doesn't express his feelings. He acknowledges them, but doesn't show them.
Anakin and Ahsoka, on the other hand, are very quick to express themselves. Obi-Wan rolls his eyes at their constant bantering, as if he's somehow more mature than they are, as if he would never do the same (even though he would, and he has).
But in those moments when Anakin gives him a wave and a friendly smile, when Ahsoka cries "Master Obi-Wan!" and throws her arms around him—he enjoys those moments, cherishes them even, and he lets it show.
And maybe that isn't so bad.