The moment Luke was waiting for. It was his time, his fight against Ken. He hadn't felt that thrill, that excitement, that feeling where there was someone who could potentially surpass him. It was somewhat similar to what Jace and K felt when they faced off against Luke in the dojo.
It was a common misconception that the people at the top of their respective fields had everything. While that was necessarily true, those individuals often shared one thing they lacked, and that was the feeling of excitement, the feeling of competition. People tend to forget that it gets lonely at the top as well.
Was he feeling threatened? Was Luke, the person who beat two legends at the same time, threatened? Of course not. Ken was exponentially better at using the Tachi but that alone usually wouldn't incite a response from Luke. There were so many weapons Luke didn't know how to use or never heard of, obviously, there would be people who were better than him at using these weapons.
