Mm, more knowledge of human behavior than speculation. He had no other reason not to bolster his padawan's confidence or reward him for his achievements, unless you'd like to suggest he was jealous of him? I don't think that's exactly true. He was entirely too frustrated with Anakin's impatience to be jealous of him. It was clearly a case of negative focus on Obi's part. Even Yoda and Mace have to remind Obi that he's not dealing with an ordinary student, and that perhaps even he might be suffering with a bit of arrogance. He believes he can force Anakin to behave by reprimanding and pointing out the negatives without an equal distribution of praise, and that he is entirely correct in this procedure. That's also why Palpatine's praise is readily consumed by Anakin and Obi's reprimands are answered with a dull, robot-like, "Yes, Master," "Yes, Master," "Yes, Master." Obi is by - the - book, which is good. Anakin is creative and improvisational, which is also good. Combined they'd make an awesome jedi, but you can't have everything.