I'm not aware of any canon answers, but it seems likely to be a combination of genuine doubts and Yoda exaggerating his reluctance.
The original plan was to train up one or both of the Skywalker twins as Jedi. But that was twenty years ago, when they had no way to predict Luke's adult character or personality.
It we take at face value Yoda's statement, "This one, a long time have I watched," he has been observing Luke using the Force and is not convinced he is good Jedi material. If all Luke does is fall to the Dark Side, and provide the Emperor or Vader with a young and powerful apprentice, it would be better not to train him at all.
When Yoda finally meets Luke, he at first pretends to be a simpleton. He then expresses his reluctance to teach Luke; this may be genuine, but it also helps to determine why and how badly Luke wants to be a Jedi. It has to be said, Luke does not distinguish himself in his interactions with Yoda, so it's understandable that Yoda continues to have doubts.
We don't know Yoda's exact state of mind, or how close he is to rejecting Luke as an apprentice. But in the end, Yoda overcomes his reluctance (real or feigned) and commits himself to training Luke.