Two comments on this:

First, to get better at programming, most people to need external input (i.e., feedback). Some of that feedback will come in a constructive form, and some may be harsh. Good Extreme Programming people learn to be tactful when giving feedback, but if someone chooses to hear any and all criticism of their code as being "this sucks", then they're cutting themselves off from a lot of valuable information, and their path up the mountain is going to be that much more difficult.

Second, if someone isn't willing to share the responsibility of helping train junior team members, then perhaps Extreme Programming isn't for them. But then perhaps lots of other things aren't for them, either. Like anything to do with being on a team. The chance to impart what you know on a junior programmer, helping mold them into a more capable programmer, is a challenge that one can learn a lot from.