Per a passage in a new Star Wars canon novel, Yoda had envisioned himself training Leia to take down the Sith, not Luke Skywalker.
When Kenobi asks Yoda to train “young Skywalker,” Yoda eagerly accepts, thinking Obi-Wan is referring to Leia.
When Obi-Wan bemoaned that Luke was their last hope, Yoda reminded him there was another.
He is disheartened to learn Obi-Wan is instead requesting for Luke be the apprentice, but Yoda obviously agrees and tells Obi-Wan to send Luke to Dagobah.